Mumps is caused by a virus which spreads by droplet infection and affects mainly children of school age and young adults. The infectivity rate is not high and there is serological evidence that 30-40% of infections are clinically inapparent. Most cases occur in the spring. The incubation period is about 18 days. A quarantine period is not necessary: contacts should be watched for the first sign of disease from the 12th to the 28th day after exposure.