Merck Manual

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Types of Infectious Organisms

Types of Infectious Organisms




Some Disorders That Can Result

Bacteria are microscopic, single-celled organisms without a nucleus.

Streptococcus pyogenes

Escherichia coli

Viruses are small infectious organisms—much smaller than a fungus or bacterium. They cannot reproduce on their own. They must invade a living cell and use that cell’s machinery to reproduce.

Varicella zoster


Fungi are neither plants nor animals. Their size ranges from microscopic to easily seen with the naked eye. They include yeasts, molds, and mushrooms.

Candida albicans


Parasites are organisms that survive by living inside another usually much larger organism (the host). They include worms and single-celled organisms called protozoa (which, unlike bacteria, have a nucleus).

Enterobius vermicularis (a species of pinworm Pinworm Infection Pinworm Infection )

Plasmodium falciparum (a species of protozoa)