The invasion and multiplication of microorganisms such as bacteria, viruses, fungi or parasites within a host organism that leads to tissue damage or disease symptoms.
Explanation
Infection begins when a pathogen enters a susceptible host through portals such as the respiratory tract, gastrointestinal tract, skin or mucous membranes. To establish infection the organism must adhere to host tissues, evade or overcome innate defences and replicate. Some pathogens remain localized at the site of entry, while others disseminate via the bloodstream or lymphatic system to cause systemic disease. Virulence factors—including toxins, enzymes, capsules and mechanisms for immune evasion—determine the severity of infection. The host mounts innate and adaptive immune responses to contain and eliminate the invading agent, producing signs such as fever, inflammation, leukocytosis and the production of antibodies. Infections can be acute, resolving within days to weeks, or chronic with persistent colonisation. They may be classified by type of organism, mode of transmission, site in the body and clinical course. Not all interactions between microorganisms and hosts cause disease; commensal colonisation is considered distinct from infection because it does not provoke damage or host responses.
Illustrative cases
Staphylococcus aureus can infect skin and soft tissues, producing abscesses or cellulitis through the release of cytolytic toxins and the formation of protective biofilms. Influenza viruses infect epithelial cells of the upper and lower respiratory tract, causing fever, myalgia and cough as viral replication damages tissues and elicits immune responses. Plasmodium falciparum infection of red blood cells leads to malaria, with cycles of fever and anemia due to rupture of infected erythrocytes. Candida albicans opportunistically infects mucosal surfaces and bloodstream in immunocompromised hosts, causing candidiasis. Nosocomial infections such as Pseudomonas aeruginosa pneumonia arise in hospital settings where patients are exposed to opportunistic pathogens and may have compromised immunity.
Understanding the mechanisms of infection and host responses guides prevention, diagnosis and treatment strategies for infectious diseases.
Related Terms: Pathogen, Infectious, Disease, Immune response, Transmission