Infection refers to the invasion and multiplication of microorganisms such as bacteria, viruses, fungi or parasites within host tissues, potentially leading to tissue damage and illness.
Explanation
An infection begins when a pathogen enters a host, overcomes physical barriers and innate immunity, and starts to replicate. Microbes may gain entry through inhalation, ingestion, direct contact, vectors or breaches in skin and mucosa. Some organisms remain localized at the site of entry, while others disseminate through the blood or lymph to distant organs. The severity of an infection depends on the pathogen’s virulence factors, the inoculum size and the host’s immune status. Toxins, enzymes and the intracellular replication of viruses can damage tissues, while the host’s inflammatory response causes many of the signs and symptoms associated with infection.
Infections can be acute or chronic and may be symptomatic or subclinical. Opportunistic infections occur when normally harmless organisms exploit a compromised immune system or disrupted microbiota. Primary infections arise in previously healthy tissue; secondary infections follow on from another illness or injury. Modern medicine distinguishes colonization, in which microbes persist without causing harm, from true infection. Diagnosis often relies on culture, nucleic acid tests or serology, along with clinical evaluation. Treatment varies from supportive care to targeted antimicrobial therapy and is guided by susceptibility patterns and the infection’s location.
Examples and notable types
- Skin infections caused by Staphylococcus aureus can produce boils, abscesses or cellulitis when bacteria enter through minor cuts.
- Influenza virus infects respiratory epithelium via inhaled droplets, leading to fever, cough and systemic symptoms.
- Urinary tract infections often result from ascending colonization by Escherichia coli, causing dysuria and urgency.
- Tuberculosis, caused by Mycobacterium tuberculosis, is a chronic pulmonary infection that can become systemic if untreated.
- Malaria is a parasitic infection transmitted by Anopheles mosquitoes; parasites invade red blood cells and cause periodic fever.
Understanding infection involves recognizing the interplay between pathogen and host, modes of transmission and factors that modulate severity. Preventive measures such as vaccination, hygiene and vector control remain critical in reducing infection burden.
Related Terms: pathogen, colonization, disease, immune response, transmission