Eighteen additional cases of mumps have been confirmed in Long Beach since the beginning of August, bringing the total number to 36 in a limited outbreak that has not spread elsewhere on Long Island, health officials said Thursday.

Epidemiologists at the Nassau County Department of Health had predicted an expansion of the outbreak on Aug. 1, when the first 18 infections were announced.

Nassau health officials have been consulting with the State Department of Health about the mumps, a highly contagious respiratory infection caused by a virus.

Despite having persisted for weeks, the outbreak, which has largely affected young adults, has spread within limited boundaries, health experts say.

“It is still geographically located in the Long Beach area,” said Mary Ellen Laurain of the Nassau County Department of Health.

She noted the infection has a relatively long incubation period of 12 to 25 days.

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The Centers for Disease Control and Prevention recommends the MMR vaccine, which generally is administered at 12 to 15 months of age and a booster between ages 4 and 6.

Nassau health officials have posted signs with bright red lettering throughout Long Beach and in nearby areas warning the public about active mumps.

“I just became aware of it today. There’s a sign about it at this beach club,” said Gaile Taubenfeld of Lawrence who was spending Thursday afternoon with her family at the Sunny Atlantic Beach Club in Atlantic Beach.

“I have six grandchildren and they all have had their vaccinations,” Taubenfeld said.

Dr. Len Horovitz, a pulmonologist at Lenox Hill Hospital in Manhattan, says mumps begins as a respiratory infection but causes its most obvious symptoms in the parotids — the two salivary glands — situated under the tongue and in front of each ear. The infection causes a painful puffy, chipmunk appearance.

The virus is transmitted through the airborne droplets of a cough or sneeze, he said, but is most often spread by hands — hand-to-hand from one person to another — and also by touching contaminated surfaces then touching one’s own face.

“The whole point of hand-washing is to wash your hands before touching your face, or before you touch someone else,” Horovitz said of a key rule that prevents infections of all kinds. “The average adult — even when you tell them not to do so — will touch their face at least once or twice a minute.”