COVID-19 is a contagious respiratory disease caused by a coronavirus, from the same family that includes SARS, MERS, and some varieties of the common cold. The virus was identified in December 2019, has spread around the world, and was declared a pandemic in March 2020. As of May 2023, there have been 688 million confirmed cases and 6.87 million deaths reported; these are inevitably an undercount. COVID-19 is widespread around the world, and many cases are not recorded.

The disease is more dangerous and more contagious than the seasonal flu, and less dangerous but more contagious than SARS and MERS. Older people and those with underlying conditions (especially respiratory problems or a weak immune system) are the most at risk of complications including death. However, young healthy individuals have also been hit seriously and even died. An infection can cause long term effects even in people who have otherwise mild symptoms.

Simple precautions — such as frequent hand washing or use of a hand sanitizer, facemasks, and when possible avoiding crowded places or keeping your distance from others — will reduce the risk of infection. A variety of restrictions have been used to combat the spread of the virus. Many countries required COVID vaccination, testing, or quarantine as a precondition for entry, and some such as Thailand require visitors to have approved health insurance. Within countries, masks have sometimes been legally required, some businesses were required to shut down at times, and various quarantine conditions were imposed.

The greatest restrictions were in 2020 and 2021; by mid-2022 many countries began to loosen these measures, and this has continued into 2023. As of May 2023, WHO has declared that the pandemic is no longer an international emergency. Some countries, such as Japan, no longer require vaccination as a condition of entry but others, such as the Philippines, do.

Common symptoms include a fever, cough, loss of appetite, and fatigue. Other less common symptoms include shortness of breath, nausea, vomiting, diarrhea, sputum production, muscle pain and loss of sense of smell. Some patients have very mild symptoms, similar to a cold, and some have no symptoms at all, but these people are still contagious. Most cases recover without special treatment, but some become seriously ill.

Serious complications include pneumonia, acute respiratory distress, and multi-organ failure leading to disability or death. As of May 2023 the fatality rate for all reported cases to date is about 1%. The rate was considerably higher early in the epidemic, but has come down as treatment methods have improved and vaccination, which reduces the risk of serious complications, has become common. Quite likely it is now below 1% for new infections.

Those most at risk of complications are the elderly and those with weakened immune systems or underlying health conditions like cardiovascular disease, diabetes, hypertension, chronic respiratory disease, and cancer. Not many cases are reported in children and most of these are mild or moderate, though a significant fraction do get pneumonia.

The time between being exposed to the virus and the emergence of symptoms (incubation period) is typically between 2 and 14 days. The disease is most contagious during the first three days of symptoms, but it can also be transmitted without symptoms. There have been recorded cases of people getting infected more than once, though reinfections are usually milder than the first infection.

Long-term effects for people who have recovered remain unclear, but there is evidence of reduced lung capacity in some recovered patients. There is also some evidence that people may develop Kawasaki disease-like symptoms after recovering from COVID-19, while the disease has also been linked with an increased risk of thrombosis, blood clots which may cause a heart attack or stroke.