Introduction to Infections: Infections: The Merck Manual for Pet Health


	Sections		Special Subjects

	Chapters		Infections

Introduction to Infections

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

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 various types of infections. Specific infections are discussed in the chapters covering the particular species and body system involved (for example, pneumonia is discussed in the chapter on lung and airway disorders).

Types of Infectious Organisms

Bacteria: Bacteria are microscopic, single-celled organisms. Examples include Escherichia coli, Salmonella species, and Staphylococcus species.

Viruses: A virus is an infectious organism that is much smaller than a fungus, bacterium, or parasite. Viruses cannot reproduce on their own; a virus must invade a living cell and use that cell's own mechanisms to reproduce. Examples include canine parvovirus, equine influenza viruses, and feline enteric coronavirus.

Fungi: Fungi are actually a type of plant. Yeasts, molds, and mushrooms are all types of fungi. Examples include Aspergillus, Candida albicans, and dermatophytes (fungi that cause ringworm).

Parasites: A parasite is an organism, such as a worm or single-celled animal, that survives by living on or in another, usually much larger, organism (the host). Examples include Dirofilaria immitis (heartworm), Ctenocephalides felis (a common flea), Sarcoptes (sarcoptic mite), Giardia, and Toxoplasma (single-celled organisms called protozoa).

Last full review/revision July 2011