Los Angeles County health officials have confirmed that at least 42 people living in L.A. have been diagnosed with mumps, most of whom are gay or bisexual men living on the west side, reports LA Times.

The news comes after officials released an alert earlier this month after the county’s department of public health stated that the “majority of these cases are among men who have sex with men, but some are women and heterosexual men with social connections to MSM cases. Most transmissions appear to have occurred at large venues such as athletic clubs, bars, theaters and nightclubs.”

According to the Centers for Disease Control and Prevention, over 3,000 mumps cases have been reported so far this year. Last year, there were 5,833 cases nationwide — the highest it's been in ten years. Many who have contracted it had no symptoms, or, flu-like symptoms along with swelling of their salivary glands, which, according to the LA Times, is a characteristic of mumps.

Some of the people who contracted the disease in LA had been vaccinated, according to Dr. Franklin Pratt, medical director of the immunization program at LA County’s Department of Public Health. It's the same vaccine most children get for mumps, measles, and rubella (the MMR vaccine), which was introduced in 1967.

The vaccine reduced cases by nearly 100 percent, so needless to say, officials are wondering why mumps have returned so drastically. According to Prat, the vaccination’s immunity might have decreased over time or the current strains of mumps are particularly strong.

“Across the country, we’re seeing mumps kind of get strength again,” Pratt said to LA Times, adding that they see nearly 13 cases per year. “This is clearly a blip.”

Prat also advised for people to wash their hands and avoid sharing drinks, especially from someone you know might have mumps. And if you have symptoms that line up with mumps, you should see a doctor immediately.

According to the alert sent out earlier this month, mumps typically begins with a few days of fever, headache, myalgia, fatigue, and anorexia followed by development of salivary gland swelling, pain, and tenderness. Gland swelling usually results in visibly swollen cheek and neck areas, which the virus is known for. In vaccinated patients, symptoms may be non-specific and in rare cases, if untreated, mumps can lead to sterility in men.

“We appreciate that you have to live your life, but do your best to limit contact with other folks until you’re past your infectious period,” Pratt said. “Getting the diagnosis is important for both advising that patient what they need to do, and also doing our very best to limit the spread.”