Coronaviruses are a family of viruses that commonly occur in humans and animals. Most coronaviruses cause mild to moderate upper-respiratory tract illnesses similar to the common cold and flu, and sometimes lower-respiratory tract illnesses, such as pneumonia or bronchitis. Most people will experience a type of common human coronavirus in their lifetime. Coronavirus Disease 2019 (COVID-19) is a novel coronavirus caused by the human body's reaction to the strain of the virus known as SARS-CoV-2. In these topics, you'll learn what COVID-19 is, who is at the highest risk of contracting it, how it's transmitted, signs and symptoms, and precautions you should take to prevent and treat it.