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Our Vaccines411 glossary provides quick and easy access to certain scientific and technical terms featured throughout the website.

Can seem to find a term you’re looking for? Feel free to request it by emailing ac.114seniccav@troppus.




Acellular vaccine
A vaccine that contains cellular material but not complete cells, specifically, antigenic or allergenic parts of cells.55
Active immunity
An individual develops active immunity when the body’s immune system produces antibodies against a specific disease, thus protecting the person from the disease. This occurs either by contracting the disease or as a result of vaccination, and generally lasts a lifetime.3
Very serious or dangerous: requiring serious attention or action, when referring to a disease.10
An adjuvant is a substance added to a vaccine to enhance the immune system’s response. Adjuvants used today make it possible to reduce the amount of antigens (weak or dead viruses or bacteria) in a vaccine. The only adjuvants used in vaccines in Canada are aluminum salts (aluminum hydroxide, aluminum phosphate, or potassium aluminum sulfate). Monitoring of vaccines over seven decades has proven adjuvants are safe. 21
The human immunodeficiency virus (HIV) is a retrovirus that infects cells of the immune system, destroying or impairing their function. As the infection progresses, the immune system becomes weaker, and the person becomes more vulnerable to infections. The most advanced stage of HIV infection is acquired immunodeficiency syndrome (AIDS). It can take 10-15 years for an HIV-infected person to develop AIDS; antiretroviral drugs can slow down the process even further. HIV is transmitted through unprotected sexual intercourse (anal or vaginal), transfusion of contaminated blood, sharing of contaminated needles, and between a mother and her infant during pregnancy, childbirth and breastfeeding.7
A condition in which the body has an exaggerated response to a substance (e.g. food or drug). Also known as hypersensitivity.
Relating to the anus and genitals (i.e. anogenital warts).11
A protein found in the blood that is produced in response to foreign substances (e.g. bacteria or viruses) invading the body. Antibodies protect the body from disease by binding to these organisms and destroying them.3
Foreign substances (e.g. bacteria or viruses) in the body that are capable of causing disease. The presence of antigens in the body triggers an immune response, usually the production of antibodies.3
An antiviral drug: used to treat diseases caused by viruses, such as herpes infections and AIDS.12
having or showing no symptoms of disease.33
A vaccine that is attenuated uses a live virus that has been made less potent through exposure to chemical or physical processes, so that the vaccination produces an immune response without causing the more severe effect of the disease.3


Bacteria are tiny, microscopic single-cell organisms that exist throughout the environment – some may be harmless while others, such as the pneumococcal bacteria, can cause serious bacterial diseases.1
BCG vaccine
BCG, or bacille Calmette-Guerin, is a vaccine for tuberculosis (TB) disease. Many foreign-born persons have been BCG-vaccinated. BCG is used in many countries with a high prevalence of TB to prevent childhood tuberculous meningitis and miliary disease.59
Birth defects

Birth defects (also known as congenital anomalies) can be defined as structural or functional abnormalities that occur to foetuses in the womb and can be detected prenatally, at birth, or sometimes may only be diagnosed later in life. Birth defects can contribute to long term mental and/or physical disabilities as well as chronic illness and death.

Although approximately 50% of all birth defects cannot be linked to a specific cause, there are some known genetic, environmental and other causes or risk factors. Some birth defects can be prevented. Vaccination, adequate intake of folic acid or iodine through fortification of staple foods or supplementation, and adequate antenatal care are just 3 examples of prevention methods.25

Some vaccines require additional shots called booster shots from time to time to increase the immune response. Booster vaccinations against tetanus and diphtheria (Td) vaccine are recommended for adults every ten years.1


(see Varicella)
Chikungunya is a viral disease transmitted to humans by infected mosquitoes. It causes fever and severe joint pain. Other symptoms include muscle pain, headache, nausea, fatigue and rash. There is no cure for the disease. Treatment is focused on relieving the symptoms.5
Cholera is an acute intestinal infection caused by the bacterium Vibrio cholerae. It is most often spread by drinking water or eating food that has been contaminated. Those who do develop symptoms usually have mild to moderate diarrhea, with or without vomiting.6
Chronic diseases
Chronic diseases are diseases that not passed from person to person. They are of long duration and generally slow progression. The four main types of chronic diseases are cardiovascular diseases, cancers, chronic respiratory diseases and diabetes.24
Cold chain vaccine
Vaccines must be continuously stored in a limited temperature range – from the time they are manufactured until the moment of vaccination. This is because temperatures that are too high or too low can cause the vaccine to lose its potency (its ability to protect against disease).42
Combination vaccine
Two or more vaccines administered in a single dose in order to reduce the number of shots given, such as the MMR (measles, mumps, rubella) vaccine.1
Community immunity
Community immunity (herd immunity) occurs when a large enough proportion of a population has developed immunity to an infectious disease that the chances of it spreading from person to person are very low. This may occur through vaccination and/or previous exposure to the illness, and the protection extends even to unvaccinated individuals within the community.1
Conjugate vaccine
Conjugate vaccine contains two compounds (e.g., a protein and a polysaccharide) that are joined in order to increase a vaccine's effectiveness.1
The introduction of microbes or bacteria into a living organism.26
COVAX is the vaccines pillar of the Access to COVID-19 Tools (ACT) Accelerator. The ACT Accelerator is a ground-breaking global collaboration to accelerate the development, production, and equitable access to COVID-19 tests, treatments, and vaccines.43
COVID-19 is a respiratory disease caused by SARS-CoV-2, a coronavirus discovered in 2019. The virus spreads mainly from person to person through respiratory droplets produced when an infected person coughs, sneezes, or talks. Some people who are infected may not have symptoms.30


Dengue Fever
Dengue fever is a disease spread to humans by mosquito bites, and is caused by one of four types of dengue viruses. It can cause severe flu-like symptoms and in severe cases can be fatal. There is no vaccine or medication that protects against dengue fever.6
Conjugate vaccine contains two compounds (e.g., a protein and a polysaccharide) that are joined in order to increase a vaccine's effectiveness.1


Ebola Virus Disease

Ebola virus disease (EVD), formerly known as Ebola haemorrhagic fever, is a severe, often fatal illness in humans.

The virus is transmitted to people from wild animals and spreads in the human population through human-to-human transmission via direct contact (through broken skin or mucous membranes) with the blood, secretions, organs or other bodily fluids of infected people, and with surfaces and materials (e.g. bedding, clothing) contaminated with these fluids.

Humans are not infectious until they develop symptoms. First symptoms are the sudden onset of fever fatigue, muscle pain, headache and sore throat. This is followed by vomiting, diarrhoea, rash, symptoms of impaired kidney and liver function, and in some cases, both internal and external bleeding (e.g. oozing from the gums, blood in the stools).

There are currently no licensed Ebola vaccines but 2 potential candidates are undergoing evaluation.23

Efficacy rate
A measure used to describe how good a vaccine is at preventing disease.7
Encephalitis is inflammation of the brain. Viral infections are the most common cause of the condition.7
The continual, low-level presence of disease in a community.3
The occurrence of disease within a specific geographical area or population that is in excess of what is normally expected.1
Epstein-Barr virus
Epstein-Barr virus (EBV), also known as human herpesvirus 4, is a member of the herpes virus family. It is one of the most common human viruses. EBV is found all over the world. Most people get infected with EBV at some point in their lives. EBV spreads most commonly through bodily fluids, primarily saliva.54
Eradication (of a disease)
The elimination of a disease from the world. To date, only one human disease—smallpox—has been eradicated. This was accomplished via a combination of surveillance and vaccination programs.13
A toxin excreted into the surrounding environment by a bacterium. For example, cholera and diphtheria bacteria produce exotoxins.58


German Measles
(see Rubella)


Hepatitis is a group of infectious diseases caused by several strains of virus. These various strains of hepatitis cause diseases known as hepatitis A, B, C, D, and E and have the potential to cause acute and chronic liver disease, and death.1
Herd Immunity
Indirect protection against disease that results from a sufficient number of individuals in a community having immunity to that disease. With enough immune individuals, the transmission of a disease can be reduced, thus limiting the potential for any one individual to be exposed to it. Herd immunity does not apply to diseases, such as tetanus, that are not spread via person-to-person contact.13
Derived from a different species.53
H. influenza (Hib)
Invasive H. influenzae disease is a contagious bacterial infection that can cause serious illness, and in young children, may result in bacterial meningitis. Immunization against H. influenzae serotype b (Hib) has significantly reduced the number of Hib cases in recent decades.1
HIB vaccine
The Hib vaccine protects against infection from the bacteria Haemophilus influenzae type b (Hib). Hib can cause serious and life-threatening infections, including meningitis, an infection of the lining that covers the brain, and septicemia, an infection of the blood. Despite its name, this disease is not the same as influenza (the flu).61
Derived from or developed in response to organisms of the same species.52
Human papilloma virus (HPV)
Human papilloma virus (HPV) is a very common sexually transmitted infection; there are more than 100 types of HPV, and it is estimated to affect more than 70 per cent of sexually active Canadian men and women at some point in their lives. It typically causes no symptoms and resolves without treatment. However, some types can result in anal and genital warts and others can lead to more serious consequences such as cervical, penile and anal cancers. Immunization helps protect against some types of HPV.2
Hybrid immunity
‘Hybrid immunity’ is the term for immunity acquired from both prior infection and vaccination.48


A state of being protected against infectious diseases by either specific or non-specific mechanisms (i.e., immunization, previous natural infection, inoculation, or transfer of protective antibodies).15
Immune system
The immune system helps the body fight disease. The immune system works to create immunity by first identifying foreign organisms such as bacteria, viruses, fungi or parasites and then, producing antibodies to defend against them (the immune response).3
Immunization – also called vaccination or inoculation – is the process that helps protect a person or animal against a disease.3
Immunogenicity is the ability of a foreign substance to enter a person's body and cause an immune response.56
Immunosuppression occurs when the immune system is compromised and thus, not able to produce an immune response in order to protect against disease. An immunosuppressed state may be the result of some illnesses such as cancer or HIV infection, or be due to the use of medications that suppress the immune system (e.g. chemotherapy). Immunosuppressed individuals should not receive live, attenuated vaccines.3
treatment or prevention of disease (such as an autoimmune disorder, allergy, or cancer) that involves the stimulation, enhancement, suppression, or desensitization of the immune system.32
Inactivated vaccine
An inactivated vaccine uses viruses and bacteria that have been killed through physical or chemical processes, and so cannot cause disease.3
The growth of a parasitic organism within the body. (A parasitic organism is one that lives on or in another organism and draws its nourishment from it.) A person with an infection has another organism (a “germ”) growing within him or her, drawing its nourishment from the person.15
Infectious period
The period of time during which an ill person may pass his or her disease to another.26
Influenza, commonly known as the flu, is very common. Millions of Canadians get the flu every year; it is a mild illness for most, but can have serious effects in some individuals, especially small children, pregnant women, seniors and those with other health problems.2
the introduction of a pathogen or antigen into a living organism to stimulate the production of antibodies.46
Intramuscular injection
Intramuscular injections are used to deliver medication deep into the muscles. This allows the medication to be absorbed into the bloodstream quickly.41
Invasive meningococcal disease
Invasive meningococcal disease is a serious disease caused by infection with the bacterium Neisseria meningitidis. The bacteria can invade the body causing serious illness, including meningitis and septicemia—an infection of the bloodstream.1
Invasive Pneumococcal Disease (IPD)
Invasive Pneumococcal Disease (IPD), which involves serious and potentially fatal infections with the bacteria Streptococcus pneumonia, affects the blood or brain. IPD can lead to brain damage, blood stream infections or death. The very young (those under the age of 5) or the elderly (those 65 and older) are most at risk of IPD. Because pneumococcal bacteria can become resistant to treatment, a preventive measure such as having the pneumococcal vaccine is particularly important.2


Japanese Encephalitis (JE)
Japanese encephalitis (JE) is a type of virus that is mainly transmitted to humans through the bite of an infected mosquito. JE occurs primarily in Asia`s rural agricultural areas, so there is a low risk for most travellers.1


Live vaccine
Live vaccine uses a live virus that has been weakened (attenuated) by exposure to chemical or physical processes, so that it produces an immune response without causing the severe effects of the disease. The herpes zoster vaccine for shingles is an example of a live, attenuated vaccine.3
Long covid
Some people still experience physical or psychological symptoms more than 12 weeks after getting COVID-19. This is called post COVID-19 condition (also known as long COVID).47
Lyme disease
Lyme disease is transmitted to humans through the bite of infected blacklegged ticks. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migraines. If left untreated, infection can spread to joints, the heart, and the nervous system. There is no evidence that Lyme disease can spread from person-to-person.29
Lymphatic Filariasis
Lymphatic Filariasis, also known as elephantiasis, is a debilitating and disfiguring disease caused by the thread-like parasitic worms, which live in the lymphatic system and can cause extreme swelling of the extremities and genitals. The disease is transmitted to humans by mosquitoes. Currently no vaccine exists for the disease.5


Malaria is a life-threatening disease caused by parasites that are transmitted to people through the bites of infected female mosquitoes.5
Marburg virus
Marburg virus disease (MVD) is a rare but severe hemorrhagic fever which affects both people and non-human primates. MVD is caused by the Marburg virus, a genetically unique zoonotic (or, animal-borne) RNA virus of the filovirus family.62
Measles, known as rubeola or red measles, is a contagious viral (i.e. caused by a virus) disease that causes a red, blotchy rash. Children are at greatest risk.2
Memory Cell
Memory cells work to defend the body against disease by quickly generating an immune response to specific invading organisms, such as viruses or bacteria, that may repeatedly attack the body. This defense is generated based on the cells’ memory of prior exposure to the specific organism.3
Meningitis is an inflammation of the meninges (the lining of the brain and spinal cord); it can result from many different types of infections caused by viruses, bacteria, fungus or other organisms.1
Meningococcus is the common name for Neisseria meningitidis, a type of bacteria spread by the exchange of respiratory and throat secretions. Meningococcal diseases can be severe and include infections of the lining of the brain and spinal cord (meningitis) and bloodstream infections (bacteremia or septicemia).2
Meningococcal Vaccines
These are a series of vaccines that target the various strains of meningoccus bacteria and the protect against the meningococcal diseases. These immunizations include: Meningococcal A, C, Y, W135 Conjugate (Quadrivalent), Meningococcal Polysaccharide A, C, Y, W135 (Quadrivalent), and recently available Meningococcal Serogroup B.1
Microcephaly is a condition where a baby is born with a small head or the head stops growing after birth.5
Multisystem inflammatory syndrome (MIS) is a rare but serious condition associated with COVID-19 in which different internal and external body parts become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal tract. MIS can affect children (MIS-C) and adults (MIS-A).60
MMR vaccine
MMR is a combination immunization against measles, mumps, and rubella; in some cases, the immunization also includes varicella, and is referred to as MMRV. Children generally receive the MMR vaccination at 12 to 15 months of age and again at 18 months or 4 to 6 years of age.1
MMRV vaccine
MMRV vaccine can prevent measles, mumps, rubella, and varicella. Most people who are vaccinated with MMRV will be protected for life.40
Monkeypox is a viral zoonosis (a virus transmitted to humans from animals) with symptoms similar to those seen in the past in smallpox patients, although it is clinically less severe.50
Monoclonal antibodies
A type of protein that is made in the laboratory and can bind to certain targets in the body, such as antigens on the surface of cancer cells. There are many kinds of monoclonal antibodies, and each monoclonal antibody is made so that it binds to only one antigen. Monoclonal antibodies are being used in the diagnosis and treatment of many diseases.51
Morbidity refers to illness or disease.3
Death, often reported as rates in specific populations.3
Mosquito Borne Diseases
Mosquito borne diseases are viral diseases which are transmitted through infected mosquitoes.5
mRNA vaccine
mRNA vaccines are a new type of vaccine that don't contain viruses or bacteria. Instead, they contain instructions that teach our cells how to make proteins that will trigger an immune response. Once triggered, our body then makes antibodies and other immune responses. These immune responses help us fight the infection to prevent us from getting sick. 36
Mumps is an infectious illness caused by the paramyxovirus virus. Although in some cases, mumps may have no symptoms, about 40% of those affected will have characteristic inflammation of one or both parotid glands, the major salivary glands located on either side of the face.1


National Advisory Committee on Immunization (NACI)
a national advisory committee of experts in the fields of pediatrics, infectious diseases, immunology, pharmacy, nursing, epidemiology, pharmacoeconomics, social science and public health. NACI has been providing guidance on the use of vaccines currently or newly approved for use in Canada to the Government of Canada since 1964.49
Non-live vaccines
This type of vaccine contains killed (inactivated) germs or their parts. It's not possible to develop disease from non-live vaccines, like the injectable polio vaccine. This is because the killed germ can't reproduce or multiply. However, the body's defence system still recognizes parts of these germs as a foreign body, stimulating an immune reaction.37


Spread of disease, which occurs in a short period of time and in a limited geographic location (i.e., neighborhood, community, school, or hospital).15


An outbreak of disease that spreads throughout the world.15
Passive immunization
A form of temporary immunity, created by giving a person disease-specific antibodies.13
A pathogen is an agent, such as an organism, that causes disease.3
Pertussis is a bacterial infection of the lungs and airways which is easily transmitted between individuals. It is also known as whooping cough, due to the serious coughing fits and “whooping” sound that may occur when catching the next breath.2
Poliomyelitis (Polio)

Poliomyelitis (polio) is a highly infectious viral disease, which mainly affects young children. The virus is transmitted by person-to-person spread mainly through the faecal-oral route or, less frequently, by a common vehicle (e.g. contaminated water or food) and multiplies in the intestine, from where it can invade the nervous system and can cause paralysis.

Initial symptoms of polio include fever, fatigue, headache, vomiting, stiffness in the neck, and pain in the limbs. In a small proportion of cases, the disease causes paralysis, which is often permanent. There is no cure for polio, it can only be prevented by immunization.22

Polysaccharide vaccine
A vaccine in which the immunogenic particles come from the carbohydrate-containing outer coatings of encapsulated bacteria.13


Quadrivalent vaccine
A vaccine that works by stimulating an immune response against four different antigens, such as four different viruses or other microorganisms. For example, Gardasil is a quadrivalent vaccine that helps protect the body against infection with four different types of human papillomaviruses (HPV).18
The isolation of a person or animal who is suspected of having a disease in order to prevent further spread of the disease.34


Rabies is a rare infection of the central nervous system that is caused by a virus. It is usually transmitted to humans through the bite of an infected mammal, and only rarely through other contact with the saliva on broken skin when no bite has occurred.2
Recombinant virus
Recombinant viruses are genetically neutered or crippled so that they cannot reproduce and cause an infectious disease once they infect a target cell.38
Required vaccines
Some countries require proof that you have received a certain vaccination before allowing you to enter the country.20
In infectious disease terms, resistance refers to the ability to resist the effects of a pathogen, or infectious organism.3
Respiratory syncytial virus (RSV)
A major cause of respiratory illness in children. The virus usually causes a common cold. But sometimes it infects the lungs and breathing passages and can cause breathing problems in infants and young children.57
Rotavirus is a virus that has been known to cause rotavirus gastroenteritis in infants. Rotavirus gastroenteritis is a highly infectious form of diarrhoea in infants and young children that can cause severe vomiting and dehydration resulting in hospitalization and can be life-threatening. Children under five years of age, especially those between six months and two years, are most vulnerable to the disease.16
Rubella, also known as “German measles”, it is caused by a virus and is highly contagious. Most people have mild symptoms including a rash, muscle aches, and low-grade fever. However, in a pregnant woman, it can result in miscarriage or fetal malformations.2


Seroconversion is the detectable presence of antibodies that have developed in the blood against an infectious organism, usually following exposure to that specific organism.3
Sexually transmitted disease (STD)
An infectious illness acquired through intimate contact with an infected person. Examples HPV, HIV and herpes. Also commonly called sexually transmitted infection (STI).13
Shingles is the name commonly used for herpes zoster, an infection that shows up as a painful skin rash with blisters, usually on part of one side of the body (left or right), often in a strip. People get shingles when the virus that causes chicken pox, varicella zoster, is reactivated in their body. The virus tends to reactivate when a person’s immune system is weakened because of another health problem.2
Smallpox is an acute contagious disease, caused by the variola virus. It is transmitted from person to person via infected aerosols and droplets from infected symptomatic people.7
Strain refers to a specific variation of an organism. Diseases such as HIV and hepatitis may have several different strains.3
Streptococcus pneumonia (S. pneumonia)
Streptococcus pneumoniae is a type of bacteria that can cause infections of the ears, sinuses or lungs.2
a pathogenic microorganism and especially a bacterium that has developed resistance to the medications normally used against it.31
A person who is susceptible to a disease is unprotected against it.3


Medical care provided remotely to a patient in a separate location using two-way voice and visual communication (by computer or cell phone).39
Tetanus is a potentially fatal disease that can affect individuals of any age. It is caused by a bacterial spore that may be found in the intestines of animals and in the soil. It has been known as lockjaw due to the painful muscle contractions it causes in the muscles of the neck and torso.2
An ethylmercury compound used as a preservative. Thimerosal was used in many killed-virus vaccines until it was recommended to be removed from most childhood vaccines in 1999. It is still used in multi-dose influenza vaccines, and can be found trace amounts (a relic of the manufacturing process) in some other vaccines.20
Tick-Borne Encephalitis
Tick-borne encephalitis is a viral disease that causes swelling of the brain, the membrane around the brain and spinal cord, and/or the spinal cord. It is transmitted through the bite of an infected tick.8
Travel vaccines
When travelling outside Canada, you may be at risk for a number of vaccine preventable illnesses. You may need additional vaccinations depending on your age, planned travel activities and local conditions of your destination. These are called travel vaccines. 19
Travellers' Diarrhea (TD)
Travelers' diarrhea (TD) affects between 30% - 70% of international travelers yearly, and can occur while traveling or after returning home. High-risk destinations include developing countries of Central and South America, Africa, the Middle East, and Asia. Infection is typically caused by ingestion of stool-contaminated food or water.4
Tuberculosis (TB) is caused by bacteria (Mycobacterium tuberculosis) that most often affect the lungs. TB is spread from person to person through the air. Tuberculosis is curable and preventable.28
Tularemia is a bacterial disease that can affect animals and humans and is most commonly found in wild animals in North America. This disease can be contracted through, the bite or lick of an infected animal, handling and cleaning of an infected animal, breathing in air or dust contaminated with bacteria or eating/drinking contaminated food. Symptoms of tularemia may include ulcers on the skin or mouth, swollen glands and painful lymph glands, sudden fever, chills, headache, diarrhea, muscle aches, dry cough, sore throat, progressive weakness, joint pain, and swollen and painful eyes.2
Typhoid Fever
Typhoid fever is a vaccine preventable infection that is caused by the bacteria Salmonella typhi. It is common in developing countries with poor sanitation and low standards of hygiene. Typhoid is most often transmitted by eating food or drinking water that has been contaminated with the feces of an infected person.6


Vaccine Shortage

A lack of availability of a certain vaccine which can occur due to: the company that manufactures it not being able to produce the vaccine fast enough to meet the demand, a company seizing to manufacture a vaccine for business reasons, a vaccine supplier not being able to send out the vaccine quickly enough, a contamination in the lab where the vaccine is manufactured, a outbreak of the illness resulting in too high of a demand, etc.

Typically the vaccine supply is not completely wiped out, but there are fewer doses than usual. During this time, doctors give vaccines first to the people who need them most. This list may include the elderly, very young children, pregnant women, people who have certain medical problems, and people who plan to travel to other countries. Other people are put on a waiting list. A shortage may last a few days to several months.27

Vaccination is the injection of a vaccine that contains killed or weakened (attenuated) organism that helps protect the body against disease caused by that organism.3
A form of a virus or disease that is slightly different from other forms.35
Varicella is known as chickenpox, is a very common and highly infectious childhood disease that is found worldwide. Symptoms appear 10 to 21 days after infection and last about 2 weeks. The defining symptom is a characteristic blister-like rash, which can cause severe irritation. Most children have a relatively mild illness, but severe illness may occur in adults and people with depressed immunity because of existing illness or because of a treatment that they are receiving (e.g. chemotherapy). The National Advisory Committee on Immunization (NACI) recommends immunization against varicella.2
A smallpox immunization technique once practiced in many locations worldwide. Variolation involved the transfer of matter from a smallpox sore into a cut in the skin of an uninfected person. The variolated person would generally experience a local reaction or mild form of the disease and thereafter be immune to smallpox. Variolation was replaced with smallpox vaccination after Edward Jenner published his findings on the use of cowpox material to induce immunity to smallpox in 1798.13
A disease that is transmitted between hosts by a vector organism such as a mosquito or tick for example West Nile virus and Lyme disease respectively.5
The ability of a pathogen to cause a disease. May also be used to describe the severity of the disease the pathogen causes.13
A virus is a microscopic organism capable of multiplying rapidly with cells and resulting in illnesses such as chickenpox, and hepatitis.3


Waning immunity
The progressive loss of protective antibodies against an antigen or disease that occurs with the passage of time. It is a crucial factor in vaccination. Booster doses of a vaccine are given when the immune response to an antigen drops below protective levels.45
West Nile Virus
West Nile is a virus that is transmitted by the bite of an infected mosquito. These mosquitoes can spread the disease to humans and other animals. Symptoms range from mild fever, headache, rash, swollen lymph glands, to more severe: loss of consciousness muscle weakness and paralysis. There is no specific treatment or vaccine for West Nile virus infection in humans. Patients are treated for their symptoms.9
World Health Organization
The World Health Organization (commonly known as WHO) is an agency of the United Nations, established in 1948, concerned with improving the health of the world's people and preventing or controlling communicable diseases on a worldwide basis through various technical projects and programs.17
Whooping Cough


Yellow Fever
Yellow fever is caused by a virus that is transmitted by the bite of an infected mosquito. It is always present in many tropical areas in South America and Africa.5


Zika is caused by a virus transmitted by Aedes mosquitoes. People with Zika usually have symptoms that can include mild fever, skin rashes, conjunctivitis, muscle and joint pain, malaise or headache. There is no specific treatment or vaccine currently available.5
Zoonotic disease
Zoonotic diseases (also known as zoonoses) are caused by germs that spread between animals and people.44


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Last Updated: February 27, 2023