A coronavirus is one of a group of RNA viruses found almost everywhere; the name comes from the fact that these viruses look like a halo (corona) when viewed under an electron microscope. In humans, they are the second leading cause of the common cold (after the rhinovirus; see below for more information); two of the human coronaviruses—OC43 and 229E—cause about 30 percent of common colds. They also infect other animals, causing respiratory infections in birds and gastroenteritis (inflammation of the digestive tract) of pigs. Compared to the majority of other viruses, they are very large in size; they also have a two-step replication mechanism—also different than most viruses. When SARS (see above) was discovered in 2002, it was found to be a new type of coronavirus.