Aug. 29, 2009 -- 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."

But infectious disease experts from both inside and outside the government say that the phrasing used by WHO raises some questions -- particularly because the existence of such a form of the disease is not a new development.

"WHO is certainly putting the fear of [God] in people with this type of release," said William Muraskin, a professor of urban studies at Queens College in New York, who is a specialist in international health. "The description by the WHO is similar to lung infections that claimed so many young people during the 1918 pandemic."

Dr. Julie Gerberding, former director of the U.S. Centers for Disease Control and Prevention, noted, "Severe pneumonia occurred in 1918 too, but we cannot confirm the pathophysiology is the exactly the same."

And Dr. Anthony Fauci of the National Institutes of Health, one of the government's preeminent figures on swine flu, told ABC News' Brian Hartman, "The severity should not be anything near what we saw in 1918 -- again, underscoring that things can change.

"But if what we're seeing now is predictive of what we'll be seeing in the fall and the winter this looks like a mild to moderate, not a very severe, pandemic."

Swine Flu May Not Be as Deadly as Past Pandemics

Indeed, many believe that the ultimate impact of the swine flu will not be as disastrous as that of pandemics of times past.

"The total mortality remains extremely low," said John Barry, author of "The Great Influenza." "And as far as the cases go, it's important to remember that while such [severe] cases have been seen, they are extremely rare."

But rare or not, the severe form of the illness is a deadly emergency. Dr. Jeffrey Boscamp, chair in pediatrics at the Children's Hospital at Hackensack University Medical Center in New Jersey, said that the lung infection triggers a syndrome called acute respiratory distress syndrome.

"The lung becomes a battleground: the virus versus all of the immunologic components that are recruited to the lung to fight the infection," Boscamp said. "The inflammation is so severe that it becomes impossible for the lung to put oxygen back into the blood.

"When oxygenation becomes impossible, other organs -- kidneys, heart, et cetera -- fail, and death can be the outcome."

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."

A spike in such severe cases could have big implications for hospitals, some fear.

"I have seen a number of these cases, with a number of deaths," said Dr. Christian Sandrock, medical director of the Intensive Care Unit at the University of California Davis Medical Center.

He added that while doctors can treat patients with much more specialized medical care now, if these very sick patients increase in numbers at hospitals, "These are the patients that are going to crush us."

Wire reports contributed to this story.