Measles and mumps are universal diseases that affect more than 90% of the population of most countries of the world. During the prevaccine era about 98% of young adults had serologic evidence of past measles infection. Although mumps infection was less prevalent, about 90% of persons were infected before they reached their 20th birthday. Before vaccines were available, measles and mumps were inevitable infections of childhood. Therefore, an assessment of the benefits and risks of immunization to prevent these diseases must also consider the consequences of naturally acquired measles and mumps infections.