Pathogen

A pathogen is any agent that can infect a host and cause disease. Pathogens may be cellular, such as bacteria, fungi, protozoa and parasitic worms, or acellular, such as viruses, viroids and prions. Only a small share of microbes are pathogens; most of those living in or on the body are harmless or helpful.

Pathogenicity is the ability to cause disease, and virulence describes how severe that disease tends to be. Neither is a fixed property of the microbe alone, because the outcome also depends on the host. A microbe that lives quietly on a healthy person, such as the yeast Candida albicans, can cause serious disease in someone whose immune defenses are weakened; such microbes are often called opportunistic pathogens. For this reason, some researchers define a pathogen by its capacity to damage a host. Pathogens do harm by invading and destroying cells, by releasing toxins, or by provoking immune reactions that injure tissue.

Diseases caused by pathogens may be communicable, passing between people directly or through air, water, food or vectors such as mosquitoes, or noncommunicable, like tetanus, which is caught from the environment. Identifying the pathogen behind an illness is a central task of clinical microbiology.