Coronaviruses are a group of viruses that cause diseases in mammals, including humans, and birds. In humans, the virus causes respiratory infections which are typically mild but, in rare cases, can be lethal. Coronaviruses are a large family of viruses that can cause respiratory illnesses such as the common cold, according to the Centers for Disease Control and Prevention (CDC). In some cases, the viruses can cause lower-respiratory tract illnesses such as pneumonia and bronchitis.