Mumps is caused by a virus, and it is passed on in the same way as a cold, meaning that it is in the mucous and saliva that come from the nose and mouth. Infection with the mumps virus causes swelling and inflammation of the parotid gland, which is a large salivary gland located right in front of and below the ear. It usually causes swelling on both sides at once, though sometimes it can be on just one side, and some people can have the infection without any swelling at all. Sometimes it can involve other salivary glands besides the parotid gland. Very often there is fever, headache and muscle aches that occur shortly before the swelling starts.

Once a person is exposed to the mumps virus, it is usually about two weeks before the person has symptoms. Once the swelling starts, mumps is contagious for as long as nine days. It is usually a very mild disease, and doesn't require any treatment. Rarely, there can be complications from mumps, including neurological problems, and inflammation of the testes in men.

Infections with mumps occur most commonly in the late winter or spring, but it is rare to see it at all nowadays, since most children are immunized against it in infancy. (The MMR vaccine that children get stands for Measles, Mumps, and Rubella. It is given at 12 to 15 months of age, and again at four to six years of age.) There are other viruses, as well as bacteria, that can cause swelling of the parotid gland, and these are different from the mumps virus. It is very uncommon to see mumps in the United States.