BOSTON -- The Harvard University School of Dental Medicine was closed after what Boston public health officials called a "probable case" of H1N1 swine flu was identified at the school.

A student who had been in contact with someone who had recently been in Mexico was believed to have the disease.

The school, which is located in Boston, would be closed Friday, city officials said at a news conference Thursday night.

In addition, city officials have requested the indefinite cancellation of classes for all third and fourth year students, as well as post-doctoral dental students.

"We're working closely with our partners in the state and Harvard to conduct a complete investigation to make sure that those who have symptoms get treated, get tested and don't infect others," said Boston Mayor Thomas Menino at a news conference.

The city also closed the Harvard University dental center, which dispenses clinic care provided by students and residents.

The male student, who was not identified, was said to be recovering.

Two probable cases of the H1N1 virus were also identified at Amherst College in western Massachusetts, according to state public health officials. Amherst did not close, but asked any student who was not feeling well to stay away from classes.

"After consulting with state and other health officials, we have decided that the campus will continue to operate regularly, while we continue to closely monitor the situation," president Anthony Marx said.

In the three cases, officials said the seasonal flu virus was ruled out, leading them to believe swine flu was the cause of the illness.

The development came after state officials said earlier Thursday that there have been no new confirmed cases of the H1N1 virus in Massachusetts. Two brothers from Lowell became the state's first confirmed cases of the virus on Wednesday.

The Lowell boys, ages 9 and 13, were recovering and expected to return to school soon.

"They're almost through with their course of recovery, because it's about a seven-day experience and one child got sick a little earlier than the other, so we should hope to make a decision by tomorrow about when they can return to school," Lowell Health Director Frank Singleton said.

"It takes seven days to recover, so usually on the eighth day you're all right but we'll check in with their physician, and with the family obviously, to see if the symptoms have obviously ceased," Singleton said.

He said no one else who had any contact with the children or their family has been tested for the virus because no one else has come down with any symptoms. He said it's unlikely they will now.

"By now, if it was going to happen it should have happened, but it hasn't," he said.

Lowell City Manager Bernie Lynch said there was no evidence the flu has spread in any way in Lowell, but all officials there will continue to monitor the situation closely.

"The health department has investigated and finds no reason to believe the virus was transmitted to anyone else," he said.

Lowell School Superintendent Chris Scott said there is a plan in place within the school system to deal with any possible outbreak, but she said everyone needs to remain calm and keep the situation in perspective.

"I think at this time level heads are prevailing. People aren't pulling their kids out of school," Scott said.

She said the boys' mother, a school department employee, had not returned to work after returning from Mexico and still hasn't. She said no other students and no teachers in the Lowell school system have exhibited flu symptoms. As for parents, she said, no one is overreacting.

Singleton said that may be because people understand that this strain of flu is relatively mild.

"It's also normal flu. It's not a deadly form of flu. If you come down with the flu, you come down with the flu. People do every year. So the fact that it's swine flu doesn't change the fact that this one, unlike what we're worried about, a very deadly strain like 1918, did not occur," Singleton said.

He said the main threat associated with this strain of the flu is that no one has any immunity to it, so there is the potential for it to spread rapidly and for large numbers of people to get sick.

"If it did, obviously there would be a major impact on commerce, on government," he said.

The boys who contracted the virus on a trip to Mexico with their family have not been identified. They are 13 and 9 years old and attend the Immaculate Conception School in Lowell. They did not attend school after returning from their trip, but the younger brother did play in a couple of Little League games before he was tested.

The 9-year-old boy played a scrimmage game last Thursday night at Shedd Park and played in the season opener on Saturday, a source told NewsCenter 5.

Gov. Deval Patrick said residents can get updated swine flu information by calling 211 or by logging on to the state's public health blog.

"I do want the public to know that we are prepared, and we want the public to be prepared," Patrick said.

The state is also awaiting test results from two possible cases of the virus in Brockton.

The Massachusetts Department of Education on Wednesday recommended any school or child care facility with a confirmed case of the virus in a student or a family member should strongly consider a temporary closure. The department also advised school staff and parents to develop a closure plan. Read DOE's Full Recommendations