Up to one-third of people infected feel no symptoms. Others have a low-grade fever, headache, weakness, fatigue, and loss of appetite starting 14 to 24 days after they are actually infected.
About a day after the onset of fever, the parotid gland near the ear begins to swell and ache – this makes chewing and swallowing painful. The body's temperature rises to 39.5°C to 40°C (103°F to 104°F).
The swelling and tenderness worsens over the next 3 days and may extend forward of the jaw and, for some, down the neck, depending on whether other salivary glands are involved. In the majority of mumps cases, both the right and left parotid glands are swollen. The fever typically lasts only 1 to 3 days but can persist for a week. The swelling of the glands tends to resolve after about one week.
That's usually as far as it goes, even in adults. 1 in 5 adolescent or adult males, however, suffers orchitis, an infection and inflammation of the testicles. This can be very painful, but it almost never results in sterility. Women can get an infection in the ovaries, but it's mild and harmless.
1 in 30 people infected get pancreatitis (inflammation of the pancreas) with vomiting and stomach ache that quickly clears up. A similar number develop hearing problems due to otitis media, which is also a temporary side effect of mumps.
Any viral disease carries some risk of severe complications like encephalitis (brain inflammation) or meningitis (inflammation of the membranes around the brain and spinal cord) during or after the initial infection. The risk of developing encephalitis with mumps is about 1 in 5,000, and for meningitis the risk is 1% to 10%. There is a very small risk of miscarriage in women who get infected while pregnant.