A single yeshiva in the Williamsburg section of Brooklyn accounted for 21 of the 31 new cases of measles this month in the city’s Orthodox Jewish community, officials said Thursday.

That brought the total number since the outbreak in October to 121 — and 108 were children under 18.

Although no one has died, there have been eight hospitalizations and one child had to be treated in an intensive care unit, officials said.

The Health Department pointed to one yeshiva — which wasn’t named — as the culprit in most of the new cases.

In December, the department announced mandatory exclusions of students attending schools in selected zip codes in Brooklyn’s Borough Park and Williamsburg neighborhoods that had not received the required number of doses of the measles vaccine.

Officials said one yeshiva ignored the order.

“This yeshiva went out of compliance with the Health Department’s exclusion order in mid-January, allowing an un-vaccinated student who had measles but had not yet begun presenting symptoms [to attend classes],” the department said in a press release.

Measles is a highly contagious disease that can cause pneumonia, encephalitis (swelling of the brain) and even death.

But measles is preventable with the measles-mumps-rubella vaccine.

Since the outbreak began, health officials have worked with elected officials, community organizations and religious leaders to conduct extensive outreach to educate residents in the affected areas about the importance of vaccination.

The effort has resulted in more than 7,000 people receiving the vaccine.

Last week, the department expanded vaccination recommendations for health providers serving the Orthodox Jewish community to include an early, extra dose of the vaccine for children between the ages of 6 months and 11 months who live in Williamsburg and Borough Park.

“As a pediatrician, I can’t stress enough how critical is to vaccinate children against measles,” said Health Commissioner Dr. Oxiris Barbot.

Early symptoms of measles include fever, cough, runny nose and red, watery eyes. Infected individuals are contagious from four days before rash onset through the fourth day after rash appearance.

The rash of measles usually starts on the face and proceeds down the body. The rash lasts several days.