The World Health Organization said Tuesday that the number of confirmed swine influenza A-H1N1 cases worldwide is close to 1,500, marking an increase of more than 400 infections since the day before. Thirty people around the world have died from the disease. In the United States, health officials are no longer recommending that schools close because of swine flu.

At a global news conference coming out of Geneva, the World Health Organization's Assistant Director-General for Health Security and Environment, Keiji Fukuda, said the organization held a virtual science meeting with investigators from several countries who have examined people with the virus, sharing clinical information with doctors worldwide.

Dr. Fukuda said the investigators have confirmed that this particular virus continues to infect mainly young people. "When investigators are looking at the average age of people getting infected, you know, this is often in the age range of people around the 20s, mid-20s - a little above, a little below," he said.

He said experts are not sure why this is the case. It might be because many of those infected are world travelers - a demographic largely composed of young people. Dr. Fukuda said there might be alternative explanations. "Because there may be something about older people, which is preventing them from being infected," he said.

But Dr. Fukuda stressed this is speculation, and that more study is needed.

In the United States, health officials are no longer recommending that schools close because of swine flu.

The Acting Director of the U.S. Centers for Disease Control and Prevention, Richard Besser, said the virus has turned out to be milder in the United States than originally feared. "When you get to situations that are approaching that of seasonal flu, then the downsides of school closure start to outweigh the benefits that you might get in your community," he said.

Last week, U.S health officials advised schools to shut down for about two weeks, if there were suspected cases of swine flu. Hundreds of schools around the United States have closed temporarily because of the virus. There are more than 400 confirmed swine flu cases in the United States, with officials saying the number likely will increase significantly as more lab-test kits are distributed across the country.

There was encouraging news from Mexico, the epicenter of the swine flu virus. Mexican President Felipe Calderon said the outbreak is waning, and that businesses should reopen soon. Universities and high schools are scheduled to resume classes on Thursday, and younger children are set to return to school in Mexico next week.