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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.


Red, raised marks on the skin that appear suddenly. It is often itchy or painful. Hives can be caused by an allergy, stress, infection or even heat and cold. Also known as hives.

Typhoid fever

Typhoid fever is a life-threatening illness caused by the bacterium Salmonella typhi. People with typhoid fever may carry the bacteria in their bloodstream and intestinal tract, some for a long time.


A once commonly used preservative in multi-dose vaccine vials, thimerosal breaks down into ethylmercury and is excreted from the body. Thimerosal has been used in vaccines, other drugs, and even contact lens solutions since the 1930s. Today, there are no…

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