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A tiny organism that multiplies within cells and causes diseases such as chickenpox, measles, mumps, rubella, and hepatitis. Antibiotics have no effect on viruses.


How much an organism, such as a virus or bacterium, can cause severe disease.

Varicella (Chickenpox)

Chickenpox is a very contagious disease caused by the varicella-zoster virus. Chickenpox causes an itchy rash that scabs over and usually lasts about a week. Scratching the scabs can cause dangerous skin infections and permanent scarring. Chickenpox can also cause…

Vaccine Safety Datalink Project (VSD)

A system that monitors vaccine adverse events. This project contains data on more than 12 million people. Researchers work to analyze this data to determine if a vaccine is causing adverse events. 

Vaccine Adverse Event Reporting System (VAERS)

VAERS is a safety monitoring system that accepts reports of adverse events that happen after vaccination. Anyone can submit a report to VAERS, but submissions do not mean that a vaccine caused the event. VAERS is one of several monitoring…


A substance that helps your body develop immunity against one or several diseases without making you sick from the disease(s).


When someone gives a vaccine to a person or animal.

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