pathogen

A pathogen, or pathogenic organism, is any organism or agent capable of causing disease in a susceptible host. Pathogens include bacteria, viruses, fungi, protozoa, helminths, and prions. Not all microorganisms cause disease; many are commensal or mutualistic and contribute to normal physiology.

Infection occurs when the pathogen colonizes the body, multiplies, or releases toxins, and interacts with host tissues. The outcome depends on the pathogen's virulence factors—such as toxins, adherence molecules, and mechanisms for invading cells—and the host's immune status and microbiota. Virulence is a measure of the capacity to cause severe disease; pathogenicity is the ability to cause disease in general.

Common routes of transmission include contact (direct or indirect), droplets or aerosols, fecal-oral, sexual contact, vector-borne transmission by arthropods, food and water contamination, and vertical transmission from mother to offspring.

Pathogens have varying host ranges; some are highly host-specific, others can infect multiple species. Some require vectors or environmental conditions to establish infection.

Detection and control rely on surveillance, diagnostics, vaccination, antimicrobial therapies, and public health measures such as hygiene, sanitation, safe food handling, and vector control. Antibiotics target bacteria but not viruses or prions; antiviral drugs, antifungals, and antiparasitics are used accordingly.

The concept is host-relative: a microorganism can be harmless or beneficial in one context and pathogenic in another, especially when host defenses are compromised or the microbiome is disrupted.