COVID-19 is the disease caused by infection with severe acute respiratory syndrome coronavirus 2, abbreviated SARS-CoV-2. It was first recognised in late 2019 and subsequently produced a global pandemic. SARS-CoV-2 is an enveloped RNA virus belonging to the coronavirus family, and infection can range from asymptomatic or mild illness to severe respiratory disease, systemic complications and death. The disease primarily involves the respiratory tract but can affect multiple organ systems because viral infection, inflammation, vascular effects and immune responses can interact across tissues.
SARS-CoV-2 is transmitted predominantly through respiratory particles released when an infected person breathes, speaks, coughs, sneezes or otherwise produces aerosols and droplets. Risk is affected by proximity, duration of exposure, ventilation, crowding, infectiousness of the source and susceptibility of exposed people. Transmission can occur before symptoms appear and from people who never develop obvious symptoms. Variants arise through viral evolution, and changes in spike protein characteristics can alter transmissibility, immune escape or susceptibility to existing antibodies. Consequently, the epidemiology of COVID-19 has changed over time rather than remaining biologically identical throughout the pandemic.
Clinical manifestations commonly include fever, cough, sore throat, nasal symptoms, fatigue, headache and muscle aches, while changes in smell or taste became particularly recognised during earlier phases of the pandemic. Severe disease can involve viral pneumonia, hypoxaemia, acute respiratory distress, thrombotic complications, cardiac injury, kidney dysfunction, neurological manifestations and other systemic effects. Risk of severe disease is influenced by age, previous immunity, vaccination, underlying conditions and other factors. Symptoms alone cannot reliably establish COVID-19 because many overlap with other respiratory infections.
Diagnosis can involve molecular tests that detect viral RNA, antigen tests that detect viral proteins, and clinical assessment in appropriate circumstances. Molecular tests are generally more analytically sensitive, while antigen tests can provide rapid results and may be useful for identifying infectious infection when viral loads are sufficiently high. Test performance depends on specimen quality, timing and the stage of infection. A negative result does not always exclude infection, particularly when testing occurs early or sampling is inadequate, so interpretation must consider exposure and symptoms.
Vaccination is an important tool for reducing severe COVID-19, hospitalisation and death, although protection against infection and transmission can decline over time and differs according to vaccine, variant and previous immunity. Other measures include ventilation, staying away from others when acutely ill, appropriate use of masks in higher-risk circumstances, testing when indicated and protecting people who are particularly vulnerable. Antiviral treatments are available for certain people at increased risk of severe disease and are most effective when started promptly within the relevant treatment window.
COVID-19 treatment depends on severity and individual risk. Mild illness may require supportive management, while severe disease can require oxygen therapy and hospital-based treatment. Specific antiviral medicines may inhibit viral replication, while selected anti-inflammatory treatments are used in certain patients with severe disease in which dysregulated inflammation contributes to tissue injury. Treatments are not interchangeable and may have contraindications or clinically important drug interactions. Antibiotics do not treat SARS-CoV-2 itself because COVID-19 is caused by a virus, although antibiotics may sometimes be appropriate for a separate bacterial infection.
Long COVID, also called post-COVID-19 condition, refers to persistent or new symptoms occurring after the acute infection and continuing for an extended period. Reported manifestations include fatigue, post-exertional worsening, cognitive difficulties, shortness of breath, altered smell or taste, sleep problems and autonomic or other systemic symptoms. Mechanisms remain an active area of research and may involve persistent immune dysregulation, tissue injury, vascular changes or other processes. Long COVID should not be confused with simply having a prolonged acute infection, and diagnosis is primarily clinical because no single laboratory test establishes the condition in all patients.
The pandemic substantially affected infectious-disease surveillance, vaccine development, genomic sequencing, public-health policy, healthcare capacity and everyday behaviour. SARS-CoV-2 continues to evolve, so recommendations and estimates of risk can change as new variants, vaccines, treatments and population immunity develop. COVID-19 is also distinct from the common cold and influenza despite overlapping symptoms. Understanding the disease requires separating the virus from the clinical syndrome, acute infection from post-acute consequences, and individual risk from population transmission patterns.
