Coronaviruses are a group of viruses that cause diseases in mammals and birds. In humans, the viruses cause respiratory infections which are typically mild, including the common cold; however, rarer forms such as SARS, MERS and the novel coronavirus causing the current outbreak can be lethal. In cows and pigs they may cause diarrhea, while in chickens they can cause an upper respiratory disease. There are no vaccines or antiviral drugs that are approved for prevention or treatment.