“We anticipate the number of cases among UW students will increase more broadly as the outbreak continues in King County and across Washington state,” the UW wrote in an email to students, faculty and staff.

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Washington’s mumps outbreak, the worst this decade, has spread to the fraternities and sororities at the University of Washington.

The UW sent a message Wednesday to students, faculty and staff saying that 20 cases of mumps associated with the Seattle campus have been reported. Those cases were concentrated in the Greek system, according to an email from the UW Advisory Committee on Communicable Diseases, and “we anticipate the number of cases among UW students will increase more broadly as the outbreak continues in King County and across Washington state.”

The UW advised those infected, or with symptoms, to stay home.

Statewide, 664 cases of mumps have been reported since October, according to the state Department of Health (DOH). That far surpasses the 53 cases in 2007, the previous high this decade. The last time the state saw such high numbers was 1977, David Johnson of DOH said.

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The mumps virus can cause fever, headache, swelling of the cheeks and jaw, muscle aches and testicular pain and swelling. In rare cases, mumps can lead to more serious complications, such as sterility. Most people recover from mumps in a few weeks.

It is spread, particularly in crowded environments, through coughing, sneezing, kissing and other close contact, such as sharing food, drinks and cigarettes.

Colleges have been hot spots for mumps outbreaks in recent years, according to the Centers for Disease Control and Prevention (CDC).

Health officials say getting vaccinated is the best defense against mumps.

The recommended dose of the MMR vaccine, which protects against measles, mumps and rubella, varies by age. For children ages 1 to 5, it’s one dose, Johnson said. The same goes for adults 20 to 59 years old. For those 5 to 19 years old, two doses are recommended.

Most people born before 1957, when mumps was more widespread, probably were infected and have natural immunity.

All 20 infected people associated with the UW had been immunized, according to a university spokesman. The mumps vaccine is about 88 percent effective, on average, according to the CDC.

About 34 percent of the cases in Washington have an unknown vaccination status, according to DOH’s Johnson. That means the infected people, mostly adults, don’t know their vaccination status, nor does the state, he said.

Among those with vaccination records, about 90 percent have the recommended immunizations, he said.

Nationally, more than 6,000 cases have been reported in 2016 and 2017, spread across 37 states, the CDC reports.

In Washington, Spokane County has reported the most cases, 283. King County is second with 240. Pierce and Snohomish counties have reported 58 and 27 cases, respectively.