Infection Health Article

Definition

Infection is the invasion and replication of microorganisms—viruses, bacteria, protozoa, or fungi—in body tissues.

Description

There are thousands of infectious agents that can cause human disease. Although the body is extraordinarily adaptive in its responses to such agents, sometimes its

preventative measures fail, resulting in disease. A subclinical infection occurs when the body's defensive mechanisms are effective, resulting in no apparent clinical symptoms. When infection persists to cause disease, it is called an acute or chronic infection.

Infectious agents

There are four major classes of organisms that infect the human body:

• Viruses: microscopic agents that consist of genetic material coding for the virus's reproduction enclosed in a protective protein coat or lipid membrane. Viruses are obligate intracellular parasites; they cannot replicate without first infecting a cell and exploiting its reproductive capabilities.
• Bacteria: microscopic prokaryotic organisms (lacking a nuclear membrane, mitochondria, and other organelles). Two major classes include gram-positive bacteria (surrounded by a protective cell wall) and gram-negative bacteria (surrounded by an outer lipid membrane).
• Fungi: eukaryotic organisms (containing distinct organelles and a nucleus enclosed by a nuclear membrane). Fungi can be unicellular (e.g., yeast) or multicellular (e.g., mold).
• Parasites: eukaryotic organisms ranging from microscopic, unicellular protozoa to macroscopic arthropods and worms.

Infectious organisms are found everywhere on Earth—in extremes of hot and cold; in acidic and alkaline environments; in air, soil, and water; in our bodies, and on our skin. The human body is colonized by numerous types of bacteria (called normal flora) that reside in the stomach, intestines, colon, upper respiratory tract, and

KEY TERMS

B cells—White blood cells responsible for the production of antibodies.

Ciliated cells—Cells with hair-like structures that help flush out foreign particles from the human body.

Complement system—Proteins that activate inflammation response and recruit white blood cells to the site of infection.

Endogenous infection—Infection caused by the normal flora of the human body.

Eukaryote—An organism whose cells contain a true nucleus bound by a membrane.

Exogenous infection—Infection caused by microbes found external to the human body.

Normal flora—Types of bacteria and other organisms that colonize the human body without normally causing disease.

Obligate intracellular parasites—Microbes that must remain inside of a cell in order to survive and replicate.

Phagocytosis—Engulfment and digestion of foreign particles and cells by phagocytic cells such as neutrophils and macrophages.

Prokaryote—A cell that contains no true nucleus or membrane-bound organelles.

T cells—White blood cells responsible for activating and controlling immune response.

on the skin. Ordinarily, normal flora aids in food digestion, protection against disease, and various other functions. Exogenous infections occur when organisms found outside of the body cause disease, while endogenous infections are caused by the normal flora colonizing sterile tissue sites.