An infectious disease is an illness caused by a pathogenic biological agent entering or multiplying within a host and interacting with the host's tissues or immune system. Infectious agents include viruses, bacteria, fungi, parasites and certain other biological entities such as prions, which cause disease through abnormal protein conformations rather than conventional cellular replication. Infection means that a microorganism or other infectious agent has established a presence in a host, but infection does not always produce symptoms or clinical disease.
A person can therefore be infected and asymptomatic, while another infected person may develop severe disease. Disease results when the interaction between pathogen and host produces sufficient tissue damage, physiological disturbance or immune-mediated injury to cause clinically significant effects. Infectious diseases range from brief self-limited illnesses to chronic infections that persist for years or decades.
Viruses are infectious agents that depend on host cells for replication because they lack the complete cellular machinery needed for independent reproduction. Examples of viral diseases include influenza, measles, COVID-19, hepatitis and HIV infection. Bacteria are cellular organisms capable of independent replication under suitable conditions, although some require particular host environments, and bacterial diseases include tuberculosis, bacterial pneumonia, cholera and meningococcal disease. Fungi can cause superficial infections such as ringworm as well as invasive disease in people with weakened immunity.
Parasites include protozoa and multicellular organisms such as helminths and can cause diseases including malaria and schistosomiasis. Prion diseases involve abnormal proteins that induce abnormal folding of other proteins and are rare but serious. Different pathogen classes therefore require different diagnostic methods, treatments and prevention strategies.
Infectious disease develops through a chain of biological events involving the pathogen, the host and the environment. A pathogen must reach a susceptible host, overcome physical and chemical barriers, establish itself in a suitable anatomical site and obtain the resources needed for survival or replication. The host responds through innate immunity, including barriers, inflammatory responses, complement and phagocytic cells, followed by adaptive immunity involving antibodies and T cells.
Symptoms may result directly from pathogen-mediated tissue damage, toxins produced by the pathogen, disruption of normal cellular functions or the host's immune response. In some infections, much of the tissue injury is caused by inflammation rather than direct destruction by the organism. Disease severity consequently depends on pathogen characteristics, infectious dose, route of entry, host age, immune status, genetics, existing conditions and other factors.
Infectious diseases can be acute, chronic, latent or recurrent depending on the pathogen and host interaction. Acute infections develop over a relatively short period and may resolve when the pathogen is eliminated or controlled, as occurs with many respiratory infections. Chronic infections persist for prolonged periods and may produce ongoing or intermittent disease, as seen with certain hepatitis viruses and some bacterial infections.
Latent infections involve periods during which the pathogen remains present but clinically inactive or difficult to detect, with the possibility of later reactivation in some diseases. Recurrent disease can reflect reinfection, incomplete clearance, reactivation or repeated exposure. These categories are not absolute because individual infections can change state over time. Understanding the temporal pattern is important for diagnosis, treatment duration and infection-control measures.
Infectious diseases are distinguished from non-infectious diseases by the presence of an infectious agent as the causal biological factor, although the boundary can become complex. Some illnesses result from infections interacting with non-infectious risk factors, while certain chronic diseases have infectious contributors without being transmissible in the ordinary sense. For example, persistent HPV infection can contribute to cancer development, but cancer itself is not normally classified as an infectious disease.
Likewise, rheumatic heart disease can follow streptococcal infection but is a later immune-mediated complication rather than direct bacterial invasion of the heart. The causal role of infection must therefore be distinguished from the clinical category of the resulting disease. This is particularly important when discussing whether a condition is infectious, communicable or contagious.
Diagnosis of infectious disease can involve clinical assessment, microscopy, culture, antigen detection, nucleic-acid amplification tests, serology and other laboratory methods. Bacterial cultures can identify organisms and permit antimicrobial susceptibility testing. Molecular tests can detect pathogen-specific genetic material rapidly, even when organisms are difficult to grow.
Antigen tests identify proteins or other components of a pathogen, while serological testing can detect antibodies generated by the host and may provide evidence of current or previous infection depending on the antibody class, timing and test characteristics. Imaging can reveal characteristic organ involvement in some infections but usually does not identify the causative organism by itself. Accurate diagnosis matters because bacterial, viral, fungal and parasitic infections require different treatments, and unnecessary antimicrobial use can contribute to resistance.
Treatment depends on the pathogen, disease severity, affected organ, host characteristics and antimicrobial susceptibility where relevant. Antiviral, antibacterial, antifungal and antiparasitic medicines target different biological processes and are not interchangeable. Supportive treatment can be equally important, including fluids, oxygen, fever control, nutritional support or management of organ complications. Antimicrobial resistance occurs when microorganisms acquire or develop characteristics that allow them to survive medicines that previously inhibited or killed them. Resistance can arise through mutation and horizontal gene transfer and can be selected by antimicrobial exposure. Prevention therefore includes vaccination where available, hygiene, safe food and water practices, vector control, appropriate infection-control procedures and responsible use of antimicrobial medicines.
Infectious diseases have shaped human history through epidemics, pandemics, population movement and changes in sanitation, medicine and public health. Improvements in clean water, sewage systems, vaccination, antimicrobial therapy, vector control and infection prevention have dramatically reduced the burden of many infections. Nevertheless, infectious diseases remain major causes of illness worldwide, and new pathogens or previously controlled diseases can emerge under changing ecological, social and environmental conditions. The study of infectious disease combines microbiology, immunology, epidemiology, pathology and clinical medicine. A comprehensive understanding requires recognising that infection is not synonymous with symptoms, that pathogens differ substantially in their biology, and that the outcome depends on a dynamic relationship between the infectious agent, the host and the environment.
