The health department said it is monitoring unusual clusters of flu cases as it works to stop the spread of the swine flu virus.

“We are now seeing a rising tide of flu in many parts of New York City,” Health Commissioner Thomas Frieden said in a statement. “With the virus spreading widely, closing these and other individual schools will make little difference in transmission throughout New York City, but we hope it will help slow transmission within the individual school communities.”

Frieden was named Friday by President Barack Obama to head the U.S. Centers for Disease Control and Prevention, where he will be faced with some immediate decisions on how to deal with the nation’s swine flu outbreak, including whether to produce a vaccine. He’ll begin at the CDC in June.

The school where the virus was first reported in the city, St. Francis Preparatory, has been cleaned and reopened, and many New Yorkers had assumed before the latest flurry of school closings that the danger of swine flu was subsiding.

But Dr. Scott Harper, an epidemiologist with the health department, said health officials weren’t surprised by the continued presence of the virus.

“It’s so unpredictable,” Harper said.

As of the weekend, there were 178 confirmed swine flu cases in New York City, Harper said, but the number of actual cases is believed to be much higher.

Health officials urged people with underlying health conditions to see their doctors if they believe they may have been exposed to swine flu. That includes people with diabetes, people whose immune systems are compromised because of certain cancer medications, pregnant women, elderly people and infants.