The World Health Organization warned Friday that doctors around the world are now reporting a severe form of swine flu that goes straight to the lungs of otherwise healthy young people -- but some infectious disease experts said the alarm could be unwarranted.

The WHO update comes in the wake of reports from some countries that as many as 15 percent of patients infected with the new H1N1 pandemic virus require extensive -- and expensive -- hospital care.

"During the winter season in the southern hemisphere, several countries have viewed the need for intensive care as the greatest burden on health services," the report said. "Preparedness measures need to anticipate this increased demand on intensive care units, which could be overwhelmed by a sudden surge in the number of severe cases."

And Dr. Greg Poland, director of the Vaccine Research Group at the Mayo Clinic, noted that the intensive care doctors he works with are in contact with other intensivists around the world who describe a bleeding, or hemorrhagic, lung infection. Poland said these doctors "are indeed seeing high viral, overwhelming viral, pneumonia, which then leads to hemorrhagic pneumonitis and severe respiratory distress syndrome; this has been requiring extraordinarily intensive therapy."

ABC News by the ABC News Medical Unit 08/29/09

Tags: H1N1, Infectious Diseases, swine flu