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Foreign substance (e.g., bacterium or virus) that is capable of causing an immune response in the body. The presence of antigens in the body triggers the immune system to act, usually producing antibodies and other immune cells.


Autism is a neurodevelopmental difference that affects how autistic people experience the world around them. Autistic people are an important part of the world. Autism is a normal neurological variation.

Attenuated vaccine

A vaccine containing a virus that is so weakened it cannot make you sick but your body can still recognize the virus and produce antibodies to fight it. Attenuated vaccines currently licensed in the United States include measles, mumps, rubella,…

Asymptomatic infection

The presence of an infection without symptoms. Also known as subclinical infection. People with asymptomatic infections can sometimes spread an illness. 


An ongoing medical condition where the bronchial tubes (in the lungs) become easily irritated. This causes wheezing, coughing, difficulty breathing and production of thick mucus. Why someone gets asthma is not yet known but environmental triggers, drugs, food allergies, exercise,…


The terms association and relationship are often used interchangeably. When two things happen close together in time or when one thing causes another thing. See causal and temporal association.


A medical condition with inflammation in the joints which results in pain and difficulty moving. It may be temporary or long-lasting.


Literally “against-virus” — any medicine capable of destroying,  weakening, or treating a virus.


A solution of antibodies against a toxin. Antitoxins give passive immunity and can help treat diseases caused by toxins.

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