pathogenicity

Pathogenicity is the capacity of an organism to cause disease in a host. It results from the interplay between pathogen traits and host defenses, and can be influenced by route of exposure, dose, microbiome, and environmental conditions. Some organisms are capable of causing disease in healthy individuals, while others are opportunistic and require compromised immunity or disrupted barriers.

Pathogenicity and virulence are related but distinct. Pathogenicity refers to the potential to cause disease; virulence describes the degree of harm caused by the organism. A pathogen can be highly pathogenic but with limited virulence in a given host, or vice versa. Virulence factors are molecular traits that enable infection, colonization, immune evasion, and tissue damage.

Common virulence factors include adhesins for attachment, invasins and secretion systems for cell entry, capsules and biofilms for protection, toxins, proteases, siderophores for nutrient acquisition, and mechanisms to evade or modulate the host immune response. Pathogenic capabilities are often encoded on mobile genetic elements and can spread between strains via horizontal gene transfer, contributing to pathogenicity islands.

Assessment traditionally uses measures such as the infectious dose (ID50) and lethal dose (LD50) in model systems, but real-world pathogenicity is shaped by host susceptibility, comorbidities, and environmental context. Pathogens are categorized as primary pathogens, capable of causing disease in healthy hosts, or opportunistic pathogens, which primarily affect compromised individuals. Understanding pathogenicity informs disease prevention, treatment, and public health surveillance.