There is a major outbreak of mumps infections in Los Angeles County, with the majority involving men who have sex with men, according to the L.A. County Department of Public Health.

The Health Department announced the outbreak yesterday. In its announcement it said 40 cases have been identified. Some involve women and heterosexuall men with social relationships with gay men.

“Most transmissions appear to have occurred at large venues such as athletic clubs, bars, theaters and nightclubs,” the Health Department said. “The majority of cases have had no documentation of complete vaccination; however, some cases were fully vaccinated.”

“Mumps is highly infectious and spreads through congregate living/social settings causing sporadic outbreaks as has been demonstrated in recent outbreaks in the U.S.”

Unvaccinated individuals are at highest risk for infection, though mumps should also be suspected among vaccinated individuals. Mumps incubation period ranges from 12-25 days, but symptoms typically develop 16 to 18 days after exposure to mumps virus.

Symptoms of a mumps infection include a few days of fever, headache, myalgia, fatigue and anorexia followed by salivary gland swelling, pain and tenderness.

Orchitis, inflammation of one or both of the testicles, is the most common complication and can occur even if you don’t experience salivary gland swelling.

Mumps can be prevented with a vaccination with two doses of mumps-containing vaccine. Such vaccinations are routinely recommended for all children and are a requirement for school attendance. Adults without proof of immunity should receive a two-shot series.