Corona viruses are a group of viruses that infect the respiratory tract of both humans and animals. Some corona viruses affect the digestive system. The virus has a crown-like projection on its surface, which is how it got its name. There are many different species of the virus. Human corona virus was discovered in 1965 and accounts for 10 percent to 30 percent of common colds. It affects all age groups and is most common during the winter and early spring. In young children and older people, human corona virus can affect the lower respiratory tract as well. Many people get corona virus for a second time within four months after having it the first time. Scientists believe this is because the antibodies created to fight the corona virus only protect a person for four months. In addition, there are many different types of human corona virus and the antibodies from one type will not protect you from another type. The incubation period of the virus -- meaning the time it is in the body before symptoms appear -- is usually two to four days, and the infection caused by human corona virus is usually mild. A relatively small amount of information is known about human corona viruses because most of them do not grow in cultured cells, making it difficult to study them.

The SARS outbreak in 2002 was believed to be caused by a new type of corona virus that was similar to the one that affects cats. Due to its contagious nature, SARS became a world epidemic, spreading to 32 countries and infecting 8,459 people. Many of the people who contracted SARS also developed pneumonia, and over 800 people died as a result of SARS.