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Protection against a disease. Immunity may be indicated by the presence of antibodies in the blood, determined with a laboratory test. There are two types of immunity, active and passive.


When the immune system is unable to protect the body from disease. This condition can be caused by disease (like HIV infection or cancer) or by certain drugs (like those used in chemotherapy). Individuals whose immune systems are compromised should…

Incubation period

The time between when you come in contact with bacteria or viruses to when you start feeling sick.

Infectious agents

Organisms capable of causing disease (e.g., bacteria or viruses).


A large cell that helps the body defend itself against disease by surrounding and destroying viruses or bacteria.


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


A group of highly contagious viruses that can cause severe diarrhea, mostly in children. Rotavirus is particularly dangerous in infants.

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