Pathogens

Pathogens are biological agents capable of causing disease in a host organism. They include bacteria, viruses, fungi, protozoa, helminths, and prions. Many organisms host commensal microbes that do not cause disease; pathogens are distinguished by their capacity to disrupt normal biological function under certain conditions. Pathogens vary in virulence, determined by factors such as toxins, adherence mechanisms, secretion systems, and the ability to evade host defenses.

Infection occurs when a pathogen reaches a suitable entry point and establishes itself, often through colonization, invasion, and replication, leading to tissue damage and symptoms. Transmission routes include respiratory droplets, contact with contaminated surfaces, fecal-oral spread, vector-borne spread, sexual transmission, and bloodborne pathways. Host factors influence disease risk: age, immune status, genetics, microbiome, nutrition, and existing diseases.

Diagnosis relies on clinical presentation and laboratory tests: culture and microscopy for bacteria and fungi; molecular methods like PCR; serology; antigen testing. Treatment depends on the pathogen group: antibiotics for bacterial infections, antivirals for some viruses, antifungals, antiparasitics; supportive care is often important; vaccines can prevent many infections. Prevention emphasizes hygiene, vaccination, safe food and water, vector control, infection control in healthcare settings. Pathogens evolve; antimicrobial resistance arises when selective pressure favors resistant strains, complicating treatment and control efforts. Prions are protein-based pathogens causing neurodegenerative diseases and have unique diagnostic and preventive challenges. Public health surveillance and rapid response are essential to detect outbreaks and limit spread.