Other meanings of Infectious disease
Medicine
An infectious disease is a disorder caused by a pathogenic microorganism—such as a bacterium, virus, fungus, parasite, or prion—that enters and multiplies within a host, disrupting normal physiological function. Transmission can occur directly from person to person, through vectors, or via environmental reservoirs. The global burden is immense: lower respiratory infections, diarrheal diseases, and tuberculosis remain leading causes of death, particularly in low-income regions1. The field of infectious diseases encompasses epidemiology, microbiology, immunology, and clinical management, and has been shaped by landmark discoveries from Pasteur and Koch to the development of vaccines and antimicrobials.
Infectious diseases are classified by the causative agent—bacterial, viral, fungal, parasitic, or prion—and by transmission route. Direct transmission includes person-to-person contact, droplet spread (e.g., influenza), and vertical transmission from mother to child. Indirect transmission occurs via fomites, airborne particles, or vectors such as mosquitoes and ticks. Zoonotic diseases, which jump from animals to humans, account for over 60% of emerging infections2. The basic reproduction number (R₀) quantifies transmissibility; measles, with an R₀ of 12–18, is among the most contagious known pathogens.
Pathogens cause disease through direct tissue damage, toxin production, or immune-mediated injury. The host's innate immune system provides immediate, nonspecific defenses, while adaptive immunity generates specific memory via B and T lymphocytes. Some pathogens, such as Mycobacterium tuberculosis and HIV, evade or suppress immune responses, leading to chronic or latent infection. The severity of disease depends on pathogen virulence factors, inoculum size, and host genetic susceptibility—for example, the CCR5-Δ32 mutation confers resistance to HIV-1 infection3.
Diagnosis relies on clinical presentation, laboratory culture, antigen or nucleic acid detection (e.g., PCR), and serology. Treatment varies by agent: antibiotics for bacteria, antivirals for viruses, antifungals, and antiparasitics. The emergence of antimicrobial resistance (AMR) threatens to render many drugs ineffective; multidrug-resistant tuberculosis and carbapenem-resistant Enterobacteriaceae are urgent threats4. Prevention includes vaccination, sanitation, vector control, and antimicrobial stewardship. The Expanded Programme on Immunization has prevented millions of deaths from measles, polio, and other vaccine-preventable diseases.
Beyond the well-known pathogens, infectious disease history includes the 1918 influenza pandemic, which killed more people than World War I, and the 1979 smallpox eradication—the only human disease eliminated by vaccination. Niche topics include the role of the microbiome in resisting colonization by pathogens, the phenomenon of pathogen-driven selection on human genes (e.g., sickle cell trait and malaria), and the use of bacteriophages as therapy, which predates antibiotics in some regions. The 2001 anthrax attacks in the U.S. highlighted bioterrorism as a modern concern, and the 2014–2016 Ebola outbreak spurred the development of rapid diagnostic tests and ring vaccination strategies5.
This article focuses on the biomedical and public health aspects of infectious diseases.
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