The mumps paramyxovirus is found in the saliva of an infected person and is transmitted either by respiratory droplets or by direct contact. The virus is present in the saliva 7 days before to 9 days after the onset of parotid gland swelling and in the urine for as long as 2 weeks after swelling begins; the 48-hour period immediately preceding the onset of swelling is probably the time of highest communicability. The incubation period ranges from 14 to 29 days (the average is 18 days). One attack of mumps (even if unilateral) almost always confers lifelong immunity.

Mumps is most prevalent in children between ages 6 and 8. Infants younger than age 1 seldom get this disease because of the passive immunity received from maternal antibodies. Peak incidence occurs during late winter and early spring.

After the virus enters the respiratory system, it replicates locally and then disseminates to target tissue, such as the CNS and salivary glands (particularly the parotid glands). A secondary phase of viremia commences when replication of the virus occurs at the target organs. The virus can travel through the bloodstream to the kidneys, potentially impairing renal function. Cell necrosis and inflammation occur with mononuclear cell infiltration. The salivary glands show edema and desquamation of ductal lining, and focal hemorrhage and destruction of epithelium may occur, leading to ductal plugging.

Meningoencephalitis is the most frequent complication of mumps and affects 10% of patients. Signs and symptoms of meningoencephalitis include fever, meningeal irritation (nuchal rigidity, headache, and irritability), vomiting, drowsiness, and a cerebrospinal fluid lymphocyte count ranging from 500 to 2,000/μl. Recovery is usually complete, but complications can occur.

Other complications of mumps may include hearing loss, orchitis, oophoritis, pancreatitis, transient myelitis, polyneuritis, myocarditis, nephritis, arthritis, thyroiditis, thrombocytopenic purpura, mastitis, and pneumonia. Epididymo-orchitis, the abrupt onset of testicular swelling and tenderness, scrotal erythema, lower abdominal pain, nausea, vomiting, fever, and chills, occurs in about 25% of postpubertal males and may cause sterility.