Mumps is a contagious illness caused by a virus that infects the salivary (parotid) glands, which are located between the ear and the jaw, and that sometimes infects other glands, particularly the testicles (orchitis). The glands become swollen and tender as a result of the virus.

Mumps can affect people of all ages, but it most commonly affects children between the ages of 5 and 19. Mumps is rare in babies younger than a year old, because they still have some protection against the virus from their mothers. It is a mild illness in children younger than 2.

Mumps is uncommon in adults because most adults have either had the illness as children or have been immunized with the measles, mumps, and rubella (MMR) vaccine. When adults do get the illness, symptoms may be more severe, and complications are more likely.