COVID-19 is a viral respiratory illness first identified in 2019 that was responsible for a pandemic between 2020 and 2023. Infection is caused by the SARS-CoV-2 virus, and transmission is possible both before symptom onset and from asymptomatic infections. Disease severity is typically mild or moderate, but infections can lead to critical illness and death. In addition, some patients develop longer-term negative health impacts after recovery from acute infection. Risk factors for more serious outcomes include older age and certain underlying health conditions. Although at reduced risk for severe illness, children have also developed severe post-infection complications requiring critical care admission. Racial, ethnic, and socioeconomic disparities have negatively affected risk for infection and the frequency of severe outcomes. Evolution of circulating SARS-CoV-2 virus is monitored by public health authorities, and the emergence of new viral variants has led to subsequent patient surges. Significant healthcare and public health advancements have occurred since initial recognition, facilitating the development of diagnostic testing and treatments. Vaccines became available in the United States in late 2020 and have become an essential COVID-19 prevention strategy.