Even as Texas health officials confirmed the first death of a United States resident with the H1N1 flu today, the U.S. Centers for Disease Control and Prevention issued new guidelines for schools responding to the virus, saying schools no longer need to close if they have suspected or actual cases of the virus.

Few details have been released, but officials said the flu victim was a woman who lived in Cameron County, along the U.S.-Mexico border, and had other, chronic health problems. The Texas Department of State Health Services said she died earlier this week.

The CDC issued the new guidelines as school closings across the country disrupted lives and the virus was deemed to be milder.

"Parents have a responsibility to look at their children in the morning and see if they're ill and not send them to school if they are," said Dr. Richard Besser, acting director of the CDC. "We feel we have enough scientific evidence that schools can be open that were closed."

The U.S. Department of Education said today that about 726 public and non-public U.S. schools in 24 states and the District of Columbia were still closed. The schools enroll about 468,000 students.

The CDC said scientists now know the virus, known as swine flu, does not contain some of the genetic factors that made it so deadly in previous pandemics. Rather, the virus mirrors what is seen in a normal flu season.

Besser also said Mexico is seeing a decline in the number of cases.

The World Health Organization (WHO) today raised its global tally of confirmed cases of the H1N1 influenza to 1,490, and said that people who are infected with the virus continue to be relatively young.

The organization said there have been 30 confirmed deaths, all but one of them in Mexico. A toddler from Mexico died in the United States.

WHO's flu director, Keiji Fukuda, said most of the new cases came from Mexico and "I expect those numbers to increase or be updated later today."

He said the increase consists of both new cases and laboratories catching up with samples that had been sent in.

Yesterday's flu tally by WHO had been 1,085 cases, with 26 deaths.

Fukuda said the average age of people getting the virus is in the mid-20s. Doctors and researchers continue to investigate why younger people are more vulnerable.

On a conference call this morning, they discussed various reasons.

Younger people just may be more susceptible to the strain of the virus. The flu is seen in people who travel, who often tend to be younger. And older people may have some immunity to the virus because of previous exposure to somewhat similar flu strains over the past several decades, Fukuda said.

It's still not known why the cases in Mexico have been more severe than the rest of the world. "It's is not something that will be found out in weeks," Fukuda said.

Outside of Mexico, the United States and Canada, Britain and Spain have the most cases. In both of those countries, the infections are related to travelers from Mexico as well as self-sustaining clusters of cases, such as schools in England, Fukuda said.

He cautioned again that while this is a mild form of influenza it could easily evolve into something worse.

"This is why we're jumping so hard on it," he said. "If it is mild and people stay healthy, that's the best outcome, but if it turns severe, we have to be prepared.

"Right now we're in a period in which there is work going on to developing a new vaccine," Fukuda said. "We are not yet at the point right now where we need to make the decision on whether to produce a new vaccine."