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Glossary of Immunization Terms

Adverse Event — A medical problem that occurs after a vaccination, which may or may not have been caused by the vaccine.

Adverse Reaction — A medical problem that occurs after a vaccination when it is assumed that the vaccine is the cause.

Antibody — A protein produced by the immune system that helps identify and destroy foreign substances that enter the body.

Antigen — A substance that causes your immune system to produce antibodies against it. A disease germ, generally a bacterium or virus.

Bacteremia — Presence of bacteria in the blood.

Clinical Trials — Testing the safety and effectiveness of vaccines before they are licensed, during which they are given to increasingly larger groups of volunteer subjects.

Communicable Disease — A disease that can spread from one person to another.

Convulsion — See Seizure.

Encephalitis — Inflammation of the brain.

Encephalopathy — An illness affecting the brain.

Epidemic — A large outbreak of disease (see Outbreak). A worldwide epidemic is called a pandemic.

Exposure — Contact with germs that cause disease. A person must be both exposed and susceptible to a disease to get sick from it.

Febrile Seizure — A seizure caused by a high fever.

Herd Immunity — Protection from disease in a community, due to a large enough proportion of the population having immunity to prevent the disease from spreading from person to person.

Immunity — Protection from disease. Having antibodies to a disease organism usually gives a person immunity.

Iron Lung — A cylindrical steel chamber that “breathes” for a person whose muscles that control breathing have been paralyzed. Some polio patients have been confined to an iron lung for life.

Local Reaction — A reaction that is confined to a small area of the body. With vaccines, a local reaction usually refers to redness, soreness, or swelling where an injection was given. (A reaction that affects the body as a whole, such as a fever or bacteremia, is called a “systemic” reaction.)

Meningitis — Inflammation of the covering of the brain or spinal cord.

Outbreak — An unusually large number of cases of a disease occurring at the same time and place, involving people who got the disease from the same source or from each other.

Paralysis — Inability to move the muscles. Paralysis usually occurs in the arms or legs, but any muscle can become paralyzed, including those that control breathing.

Schedule (or Vaccination Schedule) — The ages and/or intervals at which vaccines are recommended.

Seizure — A spell during which muscles may jerk uncontrollably, or a person stares at nothing. Usually a seizure lasts only a brief time and doesn’t cause permanent harm. A seizure can have many causes, including epilepsy or other brain disorders, or a high fever (see Febrile Seizure). Also called convulsion or fit.

Susceptible — Vulnerable to disease. Someone who has never had a disease or has never been vaccinated against it is susceptible to that disease. Opposite of immune.

Toxin — Poison.

Vaccine-Preventable Disease — Any disease for which there is a vaccine.