Doctors in Mexico are beginning to get a sense of how the new H1N1 influenza behaves, and one disturbing trend there is that it can cause severe illness in a small number of healthy young people, unlike regular seasonal flu, which is mainly dangerous to the very young, the very old and those already ill.

Whether that trend is the same in the United States isn't yet known because there have been only 57 people hospitalized here with severe illness so far and health officials only have detailed information on 26 of those, says Richard Besser, acting director of the Centers for Disease Control and Prevention. Of those, 58% had underlying conditions, including seven people with asthma.

The World Health Organization and other health officials held a conference call Thursday with doctors and medical workers in Mexico in which they reviewed every patient who has become severely ill there from H1N1. This painstaking case-by-case analysis yielded interesting results.

"We were able to differentiate two types of people at risk for severe illness," says Sylvie Briand, acting director of WHO's global influenza program.

One was previously healthy young people, the second was people — as is the case with seasonal flu — with underlying chronic conditions such as diabetes, tuberculosis, heart disease, cancer, Briand says.

In the healthy young people, "one important factor was that these people arrived very late to health care facilities" and therefore treatment came very late in the course of the disease, Briand says. That could explain the severity.

Also, those cases were early ones, before the disease was fully understood.

Most deaths resulted from severe viral pneumonia. The good news, says Briand, is that pneumonia responds to even simple, widely-available therapies such as oxygen.

In the USA, the CDC is seeing that this new flu strain seems to have a higher rates of vomiting and diarrhea associated with it than seasonal flu does.

CDC's Besser cautioned those who were ill "don't think that just because you have the symptoms of a stomach flu that you don" have influenza."

Another clue to knowing if an illness is the flu is whether it begins with a sudden onset fever of over 100.4, which almost all H1N1 flu sufferers have had as their first symptom, according to the Pan American Health Organization.

As of Friday morning, the WHO had confirmed reports of close to 2,500 people in 25 countries ill with the H1N1 virus and 44 deaths.

In the USA, the Centers for Disease Control and Prevention on Friday had confirmed a total of 1,639 cases in 43 states, with two deaths.

There continues to be no evidence that the influenza has become established outside of North America, Briand says. Most cases outside of North America "are in returning travelers or those who have had close contact with them."