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Organisms (e.g., bacteria, viruses, parasites and fungi) that cause disease.


A substance that cannot prevent or treat disease. In vaccine testing, the placebo may contain everything that is normally in the vaccine except the active ingredient. A placebo is used in clinical trials to test the effectiveness of vaccines. People…


The time before the onset of an attack or a disease.


The total number of cases of a disease within a population over a given time period.


A condition in a person that might increase the risk for a serious adverse reaction or decrease the effectiveness of a vaccine. A precaution is not a contraindication. It may not prevent a person from getting a vaccine.

Polysaccharide vaccine

Vaccines that are composed of long chains of sugar molecules that resemble the surface of certain types of bacteria. Polysaccharide vaccines are available for pneumococcal disease.

Poliomyelitis (polio)

Poliomyelitis (polio) is a viral infection that is spread person-to-person through the fecal-oral route. Children who have polio shed the virus in their excrement (poop). It is most commonly spread through unclean hands, contaminated food, and untreated water. Some people…


Pneumonia is an infection of the lungs that can cause them to fill with fluid. Bacteria, viruses, or fungus can cause it. Bacterial and viral pneumonia can be spread from person to person.


[“pe TEEK ee eye”] — Tiny reddish or purplish spots on the skin or mucous membrane. These spots do not turn white when pressed. Petechiae has many causes such as trauma, injury, and viral or bacterial infections.

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