Merck Manual

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Introduction to Infections

By

Manuals Staff

Last full review/revision Feb 2020 | Content last modified Mar 2020
Topic Resources

Microorganisms are tiny living creatures, such as bacteria, fungi, and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the hundreds of thousands of species of microorganisms invade, multiply, and cause illness in animals and people.

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Types of Infectious Organisms

Type

Description

Examples

Bacteria

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

Escherichia coli, Salmonella species, and Staphylococcus species

Fungi

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

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

Parasites

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)

Viruses

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

Many microorganisms live on the skin and in the mouth, upper airways, and intestines without causing disease. Whether a microorganism lives harmlessly in or on a pet, or invades and causes disease depends on both the nature of the microorganism and the state of the pet’s natural defenses.

This chapter provides a general introduction to the causes, signs, diagnosis, and treatment of infections.

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