Merck Manual

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Types of Infectious Organisms

Types of Infectious Organisms





Microscopic, single-celled organisms that have cell walls but lack internal structures

Escherichia coli, Salmonella species, and Staphylococcus species


Diverse, may be single-celled or multicellular organisms. Yeasts, molds, and mushrooms are types.

Aspergillus species,,Candida albicans, and dermatophytes (fungi that cause ringworm)


An organism, such as a worm or single-celled animal, that survives by living on or in another, usually much larger, organism (the host).

Dirofilaria immitis (heartworm), Ctenocephalides felis (a common flea), Sarcoptes (sarcoptic mite), Giardia, and Toxoplasma (single-celled organisms called protozoa)


Made up of genetic material that is contained in a protein coat. Viruses cannot reproduce on their own; a virus must invade a living cell and use that cell’s own mechanisms to reproduce.

Canine parvovirus, equine influenza viruses, and feline enteric coronavirus