Pathogenic

Pathogenic refers to the ability of an organism or agent to cause disease in a host. The term is derived from Greek pathos, meaning suffering or disease, and -genic, meaning producing. Pathogens include a wide range of biological agents, such as bacteria, viruses, fungi, parasites, and prions, capable of initiating disease processes under suitable conditions.

Pathogenicity—the capacity to cause disease—depends on both the microbe and the host. Microbes possess virulence factors that enable them to colonize, invade, or damage host tissues and to evade the immune response. Examples include toxins, adhesins, secreted enzymes, capsules, and mechanisms of immune modulation. The outcome ranges from asymptomatic carriage to severe illness.

Pathogens are often categorized as primary pathogens, which can cause disease in healthy individuals, or opportunistic pathogens, which cause disease when host defenses are compromised. The same organism can be pathogenic in one context and harmless in another. Diseases caused by pathogens include pneumonia from bacteria, gastroenteritis from enteric bacteria or viruses, invasive fungal infections, malaria from Plasmodium, and prion diseases.

Pathogenicity is studied through concepts such as infectivity, virulence, and pathogenesis. Virulence refers to the severity of disease a pathogen can cause; infectivity refers to its ability to establish infection. Genetic elements such as virulence genes and pathogenicity islands, often acquired via horizontal gene transfer, contribute to a microbe's pathogenic potential.

Public health emphasizes surveillance, vaccination, hygiene, and antimicrobial stewardship to prevent and control pathogenic infections. Understanding pathogenic mechanisms informs diagnosis, treatment, and the development of countermeasures against diseases caused by pathogens.