The H1N1 influenza virus is so new it's not entirely understood, South Florida doctors say. It can be as mild as a cold or, rarely, so severe that blood pressure rapidly plummets, lungs collapse and kidneys fail, sometimes causing death.

The observations come in light of a new report, published online Monday by the Journal of the American Medical Association, that says the most severe cases of H1N1 flu are happening to relatively young, healthy adults -- even though modern therapies can save most of them.

The findings seem to contradict previous beliefs, held since swine flu emerged in April, that people with underlying health conditions are most vulnerable, but may be explained by the fact that swine flu strikes young people at a greater rate. The study isn't a true snapshot on prevalence, but a JAMA editorial says it -- and a Mexican report on swine flu deaths also published Monday -- may provide clues on what hospitals elsewhere may see in coming months.

South Florida hospitals see some of the same trends. ``We're finding it in a huge variety of populations,'' said Dr. Lillian Abbo, assistant professor of infectious diseases at the University of Miami Medical School/Jackson Memorial Hospital. ``We're seeing it in young healthy adults we wouldn't expect to get as ill as this.''

The Canadian study looked at 168 swine flu patients in 38 adult and pediatric intensive care units. The patients who arrived at the hospitals with severe cases of flu had an average age of 32.2 years. Thirty-three of them died within 90 days of being admitted to the hospital.

``Our data suggest that severe disease and mortality in the current outbreak is concentrated in relatively healthy adolescents and adults between the ages of 10 and 60 years,'' wrote the study's chief author, Dr. Anand Kumar of the Health Sciences Centre and St. Boniface Hospital in Winnipeg.

The Canadian study does go on to say that modern therapies, including breathing assistance from ventilators and antiviral medicines, can prevent most flu deaths: ``With such therapy, we found that most patients can be supported through their critical illness.''

Another JAMA study also released Monday says that, in Mexico, where the disease began in March, the median age of swine flu deaths is 44 years of age. Regular flu usually is hardest on the elderly.

The fact that few of the severely ill patients in the Canadian study had underlying health conditions may be because the H1N1 virus primarily infects younger people, who tend to be healthier, the authors said. Older people -- especially those over 65, who are more likely to have underlying conditions -- appear to have some immunity because of long-previous exposure to similar viruses or to inoculations.

Patients admitted to hospitals in the Canadian study with severe symptoms usually had low levels of oxygen in the blood, fluid in the lungs and, in the most serious, multisystem organ failure, the study said. The patients worsened quickly after being admitted, most being transferred to intensive care units within two days. Most survived after intensive, long treatment.

Swine flu's toll in the United States so far this year -- about 9,000 hospitalized with 600 deaths -- is lower than the toll for regular seasonal flu, which causes about 36,000 deaths in a typical year, according to the U.S. Centers for Disease Control and Prevention.

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