More than 40 people in Los Angeles County and surrounding regions have been diagnosed with mumps, prompting public health officials Thursday to declare an outbreak and issue an alert to physicians and providers to be aware of it.

In the alert, public health officials said many of the cases were initially misdiagnosed and there was a reliance of using false negative results. It’s unclear when this occurred. Health officials also did not release the conditions of those who were diagnosed. A response from Los Angeles County public health officials was not immediately forthcoming Thursday.

But health officials said most of the patients are men who have sex with men, and the majority of the cases were linked to patients being at large venues such as gyms, bars, theaters and nightclubs. Some of the patients are women and heterosexual men who have social connections to men who have sex with men, health officials noted.

“The majority of cases have had no documentation of complete vaccination; however, some cases were fully vaccinated,” according to the alert.

Mumps is highly infectious and spreads among those living in close quarters or at social settings.

In March, seven men in L.A. County were diagnosed with mumps as part of two different clusters and occurred in men who have sex with men. It was unclear if those cases were related to the recent outbreak.

“A major factor contributing to outbreaks of mumps is being in a crowded environment,” according to the alert. “Also, certain behaviors that result in exchanging saliva, such as kissing or sharing utensils, cups, lipstick or cigarettes, may increase the spread of the virus.”

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Symptoms include fatigue, body aches, headache, loss of appetite, a low-grade fever and swelling of the salivary glands, according to health experts. In adult men, mumps can sometimes lead to sterility.

Nationwide, 2,570 people have been diagnosed with mumps from January to April of this year, according to the federal Centers for Disease Control and Prevention.

The agency reports that mumps is no longer common in the United States, but some outbreaks have been reported. Last year, there were 5,833 cases.

“Before the U.S. mumps vaccination program started in 1967, about 186,000 cases were reported each year, but the actual number of cases was likely much higher due to underreporting,” according to the CDC. “Since the pre-vaccine era, there has been a more than 99% decrease in mumps cases in the United States.”

Mumps patients are contagious from two days before through five days after onset, according to the CDC.

Public health officials said routine vaccination is the most effective way to prevent the disease. California requires that all children entering kindergarten receive two doses of the MMR vaccine. As of July, 1 2016, only children with medical proof can be exempt.