COVID-19

COVID-19 is the illness caused by a coronavirus that emerged in late 2019 and spread worldwide. For many people it behaves like a bad cold or flu — fever, cough, sore throat, tiredness, and sometimes a striking temporary loss of smell or taste — but in a minority it travels deep into the lungs and causes a serious pneumonia, which is what made it so dangerous on a large scale.

The disease is caused by the virus SARS-CoV-2, which infects cells lining the airways and lungs and spreads mainly through respiratory droplets and fine aerosols. Severity ranges enormously: from no symptoms at all, through mild upper-airway illness, to a viral pneumonia in which the alveoli fill with inflammatory fluid, oxygen levels fall and, in the worst cases, the syndrome of severe lung injury called ARDS develops. Older age and chronic illness raise the risk of severe disease.

Management spans the spectrum: rest and monitoring for mild cases; supplemental oxygen, corticosteroids and antiviral or other agents for those needing hospital care; and intensive support such as high-flow oxygen or mechanical ventilation for the most severe. Vaccines and prior infection reduce the chance of severe outcomes, and the picture has shifted over time as the virus evolved and population immunity grew.

Some people experience lingering symptoms for weeks or months after the acute infection, often called long COVID; this is an area of ongoing research, and not every persistent symptom after the illness is necessarily caused by the virus.