Glossary of Terms

Common terms that you may encounter on the site

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Adjuvant
An agent added to some vaccines that enhances the immune system’s response to vaccine antigens. Adjuvants used in vaccines include aluminum salts and, in Canada and the European Union, squalene, which is a fish-oil derived material.
Adverse event
A possible side effect resulting from a vaccination.
Antibody
A protein generated by the immune system in response to the presence of a specific antigen. Antibodies attach to that antigen to help counter its effects.
Antigen
A substance that provokes the immune system to generate antibodies against it.
Antigen-presenting cell
A white blood cell that recognizes antigens, binds to them, ingests them, breaks them down, and then presents pieces of them to T cells.
Antigenic drift
Change in the genetic makeup of a virus, especially an influenza virus, that leads to a new strain of the virus. Antigenic drift causes the virus’s outer surface to appear different to a host previously infected with the ancestor strain of the virus. Antibodies produced by previous infection with the ancestor strain cannot effectively fight the mutated virus, and disease often results.
Antigenic shift
The process by which different influenza A viruses combine to result in a new subtype of virus. Antigenic shift may result in global disease spread, or pandemic, because humans will have few or no antibodies to fight the virus.
Antitoxin
Antibody that neutralizes a toxin. Diphtheria antitoxin, for example, is derived from the blood of horses immunized with diphtheria toxin and is given, along with antibiotics, to treat patients with clinical diphtheria.
Attenuation
The process of weakening a pathogen. Attenuation may be achieved in a variety of way: by exposing the pathogen to heat or chemicals, for example, or by passing the pathogens through a growth medium many times. The goal of attenuation in virology is to produce an antigen that is capable of stimulating an immune response, and thus creating immunity, but not causing disease.
Bacteria
A group of microscopic single-celled organisms. Some bacteria are capable of causing disease in humans and other organisms.
Booster
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.
Epidemic
A collection of infections by a specific disease that exceeds the number of expected infections during a particular period of time.
Epidemiology
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, whereby new cases could be detected and potentially exposed individuals vaccinated in order to halt the spread of the disease.
Exotoxin
A toxin excreted into the surrounding environment by a bacterium. For example, cholera and diphtheria bacteria produce exotoxins.
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. Types A-G are caused by viruses. 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, 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.
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.
Immunity
The state of an individual’s existing resistance to a pathogen, whether conferred by a previous infection (“natural immunity”), or via vaccination.
Immunization
A procedure for creating resistance to a pathogen. Such procedures include vaccination and treatment with antibodies.
Immunogen
A substance able to provoke 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.
Inoculation
The introduction of material into a person in order to provoke immunity to a disease. “Inoculate” is often used to refer to the historical practice of variolation. (See Variolation.)
Killed vaccine
See Inactivated vaccine.
Lymph
This clear fluid, derived from blood plasma, contains white blood cells.
Morbidity
In epidemiology, this refers to the incidence rate of a particular disease.
Mortality
In epidemiology, this refers to the death rate from a particular disease.
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.
Pathogen
An agent, such as a bacterium, virus, worm, or fungus, that causes disease.
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.
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 separating out solids.
Sexually transmitted 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 have hypothesized that SIVs led to the transmission of HIV (human immunodeficiency virus) in humans. Evidence suggests that 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) 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 found trace amounts (a relic of the manufacturing process) in some 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.
Toxin
Poisonous substance capable of causing disease. Toxins are generated by cells or organisms; toxins generated by bacteria are often 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.
Toxoid
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 vaccine is widely taken by a target population. 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.
Vaccine
Antigenic material in the form of weakened, killed, or modified pathogens, introduced to the body to induce immunity against a particular pathogen.
Variolation
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.
Virulence
The ability of a pathogen to cause a disease. May also be used to describe the severity of the disease the pathogen causes.
Virus
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 in order to reproduce itself. Viruses are ultramicroscopic, meaning that 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.