Infectious

Infectious describes a characteristic of agents or diseases that can be transmitted from one organism to another, often leading to infection. An infectious disease arises when a pathogenic organism or toxin invades the body and disrupts normal function. Pathogens include bacteria, viruses, fungi, parasites, and prions. Not all infections cause disease; some people can carry pathogens without symptoms and still spread them to others (asymptomatic carriage).

Transmission occurs through multiple routes, including direct contact, indirect contact with contaminated objects, droplets from coughing or sneezing, airborne spread of small particles, vectors such as mosquitoes or ticks, and ingestion of contaminated food or water. The ease with which an infection spreads is described as its contagiousness; all contagious diseases are infectious, but some infections are less easily transmitted.

Public health strategies aim to limit transmission and reduce disease burden. These include vaccination, antimicrobial and antiviral therapies, and measures such as hand hygiene, isolation or quarantine, sterilization of equipment, safe food and water practices, and vector control. Surveillance and rapid diagnosis also play critical roles, as do efforts to combat antimicrobial resistance.

Common infectious diseases include influenza, tuberculosis, HIV/AIDS, malaria, and COVID-19. The category also encompasses emerging infections and zoonoses, where pathogens move from animals to humans.