City health officials confirmed 20 more cases of swine flu Monday -- bringing the total number of city residents affected to 28, with another 17 probable cases still being looked into.
Officials say they are also checking out possible illnesses at PS 177 in Fresh Meadows, Queens, which is not far from Saint Francis Prep -- the school where the virus originally surfaced.
During a press conference Monday, Mayor Michael Bloomberg said that all of the cases were isolated within the Saint Francis Prep High School community in Queens. It is believed that more than 100 at the school will become ill.
"This remains an isolated incident. There is no reason for anyone outside of the St. Francis community to stay home or change their daily routine unless they are sick," Bloomberg said. "Everyone should go to work and go about their day. And no one who is planning to visit New York should change their plans. The city is safe and open for business."
"We have no severe suspected cases," said NYC Department of Health Commissioner Dr. Thomas Frieden. "That doesn't mean there won't be any in the future, but as of now we don't have any. We call every intensive care unit in the city everyday."
Officials said none of the cases appear to be severe.
Saint Francis Prep in Fresh Meadows will remain closed through Tuesday, as cleaning crews sanitize the school.
Six of those who contracted the flu had traveled to Mexico for spring break.
Some 150 students at the school have reported flu-like symptoms. Testing is underway to determine if there are any more cases.
One student who spoke to NY1 is still awaiting his test results.
"I don't know what I have as of now, but I was telling my mom this is really weird," said the student. "It feels like the flu, but it's unlike anything I felt before. It was a weird feeling. But this was before I knew anything, so it's not like I was being a hypochondriac or anything."
In the past three days, there have been no new cases, which the mayor says is very good news.
In the Bronx, a day care center suspected of a having a possible swine flu outbreak among six children turned up negative in diagnostic tests.
Face masks are not necessary for the general public unless you are coming into contact with affected students at St. Francis.
The mayor said basic hygiene is crucial to containing the outbreak.
"To members of the public, if you have fever and cough or sore throat, stay home," the mayor said. "Do not go to work or to school. Wash your hands frequently and as always, when you cough or sneeze, cover your mouth and nose. This is how to reduce the risk of transmitting this illness."
He added the city, state and federal governments are working together to handle the outbreak in New York.
The latest figure raises the number of cases nationwide to 40, the Centers for Disease Control says.
The World Health Organization says it is "very concerned" about the disease's spread and have raised the pandemic alert to a level 4, which is two steps down from a full blown pandemic.
Officials say they are moving aggressively to control the outbreak, and Homeland Security Secretary Janet Napolitano downplayed her declaration of a health emergency as standard operating procedure.
The CDC is urging people to forgo all nonessential travel to Mexico.
The acting director of the Centers of Disease Control and Prevention says the recommendation is part of the government's aggressive response to the threat.
"Control of an outbreak of infectious disease is a shared responsibility," said CDC Acting Director Dr. Richard Besser. "There are things that individuals need to do, there are things that communities need to do, there are things that the government needs to do. And it's important that individuals realize they have a key role to play in reducing their own likelihood of getting infected."
There are now 40 confirmed cases of swine flu in the U.S. in five states, including New York, Ohio, Kansas, Texas and California.
President Barack Obama says the federal government is closely monitoring the situation.
"The Department of Health and Human Services has declared a Public Health Emergency as a precautionary tool to ensure that we have the resources we need at our disposal to respond quickly and effectively," he said.
The president added that the nation's capacity to deal with this kind of challenge rests heavily on the scientific and medical community and is another example of why the U.S. cannot allow itself to fall behind.
Work is underway on a vaccine, and 12-million doses of the anti-viral drug Tamiflu have been made available to New York and other states.
So far the disease in the United States seems less severe than the outbreak in Mexico.
Meanwhile, state officials are telling parents to prepare for the possibility of mass school closings, though Governor David Paterson says parents should feel safe sending their kids to school.
Swine flu cases have also been reported in Ohio, Kansas, Texas and California.
Swine flu is no longer just in North America. Cases have now been reported in Europe, as well.
Spain's Health Ministry has confirmed the country's first case of the flu. Another 20 people there are suspected of having the disease.
The confirmed case was found in a man who had recently traveled to Mexico.
The European Union is advising citizens to hold off on traveling to affected parts of Mexico and the U.S.
Suspected swine flu cases have also been reported in New Zealand, Canada and France.
Countries throughout Asia are checking incoming passengers for signs of fever as they enter the country. China, Russia and Taiwan are considering plans to quarantine sick travelers.
Health officials say the flu may have killed nearly 150 people in Mexico. As a result, the country has shut down all schools.
Mexican President Felipe Calderon has assumed new powers to isolate infected people, including inspecting homes and travelers.
Millions of masks have been handed out and people with flu symptoms are being urged to stay home and hundreds of public events have been cancelled.