Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites.

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Typically, these are bacteria, viruses, fungi, or worms/ helminths. Under normal circumstances, when the immune system of the host is fully functional, disease symptoms may not develop. If the host immune system is compromised, or the infectious agent overwhelms the immune system, an infectious disease ensues. Most infections are caused by bacteria, viruses, protozoa, helminthes, rickettsia, and fungi.