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Glossary of Terms

Below are definitions of some terms you may encounter on our site.

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  • Adjuvant

    This is something added to some vaccines to help the immune system respond better to the vaccine. Aluminum salts and squalene (a material from fish oil used in Canada and the European Union) are examples of adjuvants.

    Adverse event

    This is a possible side effect that can happen after getting a vaccine.


    A protein made by the immune system when it detects something foreign, like a virus or bacteria. Antibodies stick to the foreign material to help stop it from causing harm.


    A substance that makes the immune system produce antibodies against it because it sees it as foreign. This substance could be a protein, a lipid (fat), or a sugar strand on the surface of an invading microbe.

    Antigen-presenting cell (APC)

    A type of white blood cell that finds foreign substances, eats them, breaks them down, and then shows (presents) parts of them to other immune cells to help the body defend itself.

    Antigenic drift

    This happens when the proteins on the surface of a virus, like the flu virus, change slightly over time. These small changes can make the virus look different to the immune system of someone who had the flu before, making their previous antibodies against the antigen less effective at fighting off the virus.

    Antigenic shift

    A big change that happens when different flu viruses mix and create a new flu virus type. This can lead to a worldwide spread of the flu because people might not have any immunity to the new virus.


    A type of antibody that fights off toxins. For example, the diphtheria antitoxin comes from horses given the diphtheria toxin. The horses make antitoxins, and those antitoxins are used to treat people with diphtheria, along with antibiotics.


    This is how a germ is made weaker, so it can't cause disease. It can be done by heating the germ, treating it with chemicals, or growing it many times. The goal is to make the germ weak enough to not cause disease, but strong enough to be included in a vaccine that can teach the immune system to fight the germ without making the person sick.

  • Bacteria

    A group of microscopic single-celled organisms. Some bacteria are capable of causing disease in humans and other organisms.


    A vaccine dose given to stimulate the immune system’s memory response.

  • Cell Culture

    See Tissue Culture.

    Conjugate Vaccine

    A vaccine in which an immunogenic particle from a target pathogen is bound to a carrier molecule. Conjugate vaccines include certain pneumococcal and meningococcal vaccines.

  • Diphtheria

    An infectious disease caused by the bacteria Corynebacterium diphtheriae. Diphtheria is transmitted from person to person, usually via respiratory droplets. The most notable feature of diphtheria infection is the formation of a thick gray substance called a pseudomembrane over the nasal tissues, tonsils, larynx, and/or pharynx.

    DNA Vaccine

    A vaccine made from sections of a pathogen’s DNA. Once injected, the host creates proteins that stimulate an immune response against the pathogen. Several veterinary DNA vaccines are licensed in the United States.

  • Elimination (of a disease)

    The reduction of cases of a disease so that it no longer naturally exists in a particular region. For example, measles is considered eliminated in the United States (even though a few cases per year are still imported from foreign countries) because the disease does not continuously circulate there.

    Endemic (disease)

    A disease that constantly circulates in a particular population or geographic region. Measles, for example, is considered endemic in Great Britain because it continues to circulate freely there. In contrast, measles has been eliminated in the United States, and so is no longer endemic there.


    A collection of infections by a specific disease that exceeds the number of expected infections during a particular period.


    The study of diseases and how they occur at a population level. Epidemiologists study the frequency and distribution of diseases among populations.

    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, where new cases could be detected and potentially exposed individuals vaccinated to halt the spread of the disease.


    A toxin excreted into the surrounding environment by a bacterium. For example, cholera and diphtheria bacteria produce exotoxins.

  • Fomite

    An inanimate object capable of transmitting a disease when contaminated with an infectious agent.

  • Gamma globulin

    Antibody-containing blood protein. Gamma globulin may be given as a treatment for a particular disease. The expectation is that disease-specific antibodies in the gamma globulin will fight the disease agent before it causes illness, or that the gamma globulin will reduce the severity of illness.

  • Hepatitis

    Any one of several forms of a disease causing inflammation of the liver. Viruses cause types A-G. Vaccines exist for Hepatitis A (usually caused by food contaminated with feces containing the virus) and Hepatitis B (usually spread by sex with an infected partner or the sharing of needles with an infected person).

    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, limiting the potential for 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.

  • IgG

    Antibody molecules secreted by B cells. IgG is the most abundant antibody found in gamma globulin.

    Immune System

    The collection of organs, cells, and molecules that make up an organism’s response to a disease threat or foreign substance.


    The state of an individual’s existing resistance to a pathogen, whether conferred by a previous infection (“natural immunity”), or via vaccination.


    A procedure for creating resistance to a pathogen. Such procedures include vaccination and treatment with antibodies.


    A substance that provokes an immune response. (“Immunogenicity” refers to an immunogen’s ability to provoke such a response under particular circumstances.)

    Inactivated Vaccine

    A type of vaccine in which the vaccine pathogen (a viral or bacterial) is killed or otherwise altered so that it cannot cause infection, but can still provoke an immune response. Immunity generated in response to inactivated vaccines typically does not last as long as immunity from live, attenuated vaccines.

    Infectious Disease

    An illness caused by a pathogen that invades an organism.


    The introduction of material into a person to provoke immunity to a disease. “Inoculate” is often used to refer to the historical practice of variolation. (See Variolation.)


    The restriction of movement of a person who tests positive or shows signs/symptoms of an infectious disease for a length of time necessary for them to not be contagious.

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  • Killed Vaccine

    See Inactivated Vaccine.

  • Lymph

    This clear fluid, derived from blood plasma, contains white blood cells.


    A type of white blood cell (immune cell) primarily found in lymph.

  • Morbidity

    In epidemiology, this refers to the incidence rate of a particular disease.


    In epidemiology, this refers to the death rate from a particular disease.

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  • Pandemic

    An epidemic that occurs in several geographic regions. (Note that a disease need not have a high death rate to cause a pandemic.)

    Passive Immunization

    A form of temporary immunity, created by giving a person disease-specific antibodies.


    An agent, such as a bacterium, virus, worm, or fungus, that causes disease.


    A vaccine-preventable .

    Plasma (in blood)

    The protein-containing fluid part of blood. Red blood cells and other materials are suspended in plasma.

    Polysaccharide Vaccine

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

  • Quarantine

    Restriction of movement and contact of a person after a potential exposure to an infectious agent for a period of time to ensure they do not pass the disease to others. The length of time depends on the infectious agent, and is usually the average time from exposure to detection of the disease. If a person is deemed not infected at the end of the quarantine, they may resume their activities of daily living.

  • Ring Vaccination

    A vaccination strategy in which likely contacts of an infected person are targeted for vaccination. This strategy contrasts with universal vaccination, in which large populations are targeted for vaccination. Ring vaccination was an essential part of the successful campaign to eradicate smallpox.

  • Serum

    The liquid obtained after allowing blood to clot and removing the solids.

    Sexually Transmitted Infection/Disease (STD)

    An infectious illness acquired through intimate contact with an infected person. Examples include syphilis, gonorrhea, HPV, HIV, herpes, Chlamydia, and others. Also commonly called sexually transmitted infection (STI).

    SIV (Simian Immunodeficiency Virus)

    Simian immunodeficiency viruses are retroviruses found in primates. Researchers hypothesized that SIVs led to the transmission of HIV (human immunodeficiency virus) in humans. Evidence suggests both HIV-1 and HIV-2 originated in Africa, with HIV-1 being a relative of a SIV that infects wild chimpanzees, and HIV-2 being a relative of one that infects the sooty mangabey. It is likely that the first human cases of HIV resulted from contact with an infected chimpanzee or monkey.

    Strain (Of a Virus)

    A variant of a virus. Strains of a certain type of virus are usually differentiated by minor genetic changes. They retain enough similarities, however, to belong to the same species.

    SV40 (Simian Virus 40)

    A polyomavirus (not to be confused with poliovirus) is found in both monkeys and humans. Early versions of the polio vaccine were found to be contaminated with SV40, which led to widespread alarm when tests showed that hamsters infected with the virus grew cancerous tumors. Long-term epidemiological studies, however, found no difference in the incidence of cancer among those who did and did not receive the contaminated vaccine. No evidence to date suggests that SV40 can cause disease in humans.

  • Thimerosal

    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 trace amounts (a relic of the manufacturing process) in other vaccines.

    Tissue Culture (Cell Culture)

    A collection of living cells that provides a cultivation medium for viruses, bacteria, and other organisms. The propagation of viruses in tissue culture was an important milestone in studying viruses and creating vaccines. The team of John Enders, Thomas Weller, and Frederick Robbins won a Nobel Prize in 1954 for developing tissue culture techniques in their study of poliovirus. Tissue culture is often used interchangeably with cell culture, though it can refer to the use of pieces of tissue, rather than collections of cells, as a growth medium.


    Poisonous substance capable of causing disease. Toxins are generated by cells or organisms; toxins generated by bacteria often are the actual cause of the disease associated with those particular bacteria. Tetanus, for example, is not directly caused by the presence of Clostridium tetani bacteria, but by tetanospasmin, the toxin they produce.


    A toxin modified to invoke an antibody response, but not capable of causing disease. Immunizations for diphtheria and tetanus use toxoids.

  • Uptake (Of a Vaccine)

    On a population level, the coverage level or use of a vaccine. A high uptake rate indicates that a target population widely takes a vaccine. A low uptake rate means the vaccine is not widely taken.

  • Vaccination

    Introduction into an organism of a material designed to provoke an immune response that will provide protection from a related disease agent.


    Antigenic material in the form of weakened, killed, or modified pathogens, introduced to the body to induce immunity against a particular pathogen.


    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, however, carried the risk of severe infection and death. 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.

    Venereal Disease

    An older term used for .


    The ability of a pathogen to cause a disease. May also be used to describe the severity of the disease the pathogen causes.


    An infectious particle consisting of DNA or RNA surrounded by a protein coat. A virus is not a living thing, and requires a living host cell to reproduce itself. Viruses are ultramicroscopic, meaning they are too small to be seen by anything other than a powerful electron microscope.

    Virus-Like Particle (VLP)

    Antigenic material from the outer protein coat of a virus. For example, the human papillomavirus vaccine uses HPV VLPs, assembled in yeast cells, to stimulate the immune system to protect against the virus.

  • Whooping Cough

    See Pertussis.

  • X-ray

    An image of the inside of structures (primarily bodies) taken with specialized equipment.

  • Yersinia pestis

    Bacteria that cause .

  • Zika Virus

    A mosquito-borne virus that causes .