More than 300 cases of mumps have surfaced in May in the Maritimes — 56 in New Brunswick alone — and health officials say those in their late teens and early 20s are most at risk.

Jeannette Macey, the head of Disease Surveillance with the Public Health Agency of Canada, said the outbreak stems from waning immunity within that age group.

When mumps vaccines were first given, they came in a single dose. Since the 1990s, they have come in two doses. Young people who did not return for a second "booster" vaccine — or missed vaccination altogether — are the ones fostering the new outbreak.

"It actually takes a number of years for a few people missed in each age group to accumulate to the point where you actually have enough of a subpopulation to feed an outbreak, and this is exactly what we've seen here," Macey said.

The first cases surfaced in Nova Scotia in February, then the viral disease began cropping up in New Brunswick in early April and in P.E.I. a week or two later.

Nova Scotia has been hit hardest: it had reported 272 cases by May 18 and more than 100 hospital workers in the province were forced to take paid leave after testing positive for exposure to mumps.

New Brunswick has reported 56 cases so far. There are just two in P.E.I., both students who attended Dalhousie University in Halifax.

Allison Grenon-Bent, a 27-year-old who lives in Fredericton, is one of those who was recently diagnosed with the disease.

"At first I thought I had the flu actually, but I never got the runny nose or the cough or anything, so I just sort of waited it out," Grenon-Brent said.

Grenon-Bent started feeling sick around Mother's Day, and after five days, she went to the hospital.

"It was getting to the point where I was starting to get these bumps on my neck and on my face and the Tylenol wasn't working anymore, so I finally caved and I went up and found out I had the mumps," said Grenon-Bent.

"They still sent me home anyway, and they said they couldn't do anything about it. Now I'm just sitting at home, waiting for it to go away."

Nova Scotia is offering vaccinations to 30,000 health-care workers provincewide.

New Brunswick has yet to decide whether any new vaccination program is necessary.