I don't know why people are so determined to ignore this. As someone who's had pneumonia, trust me, it's not an experience you want to have. I was lucky enough to have a bacterial strain that responded to antibiotics. Please, see a doctor if you have any doubts:

The World Health Organization urged doctors Friday to treat suspected swine flu cases as quickly as possible with antiviral drugs, warning that the virus can cause potentially life-threatening viral pneumonia much more commonly than the typical flu, sometimes in relatively young, otherwise healthy people.

"It's not like seasonal influenza," said Nikki Shindo, a medical officer in the WHO's Epidemic and Pandemic Alert and Response Department. "It can cause very severe disease in previously healthy young adults."

Shindo's comments came at the conclusion of a special three-day meeting in Washington of more than 100 experts from around the world. The WHO called the meeting to review the latest research on the new H1N1 virus and to revise guidelines for treating the infection.

Unlike the seasonal flu, Shindo said, the virus appears more likely to travel deep into the lungs, where it can cause viral pneumonia. Such a condition can cause severe lung damage and a life-threatening condition known as acute respiratory distress syndrome.

"Remarkably different is this small subset of patients that presents very severe viral pneumonia," Shindo said.

Shindo noted that some hospitals in Australia and New Zealand were severely strained by seriously ill swine flu patients during their recently ended winter.

"This disease overwhelmed emergency rooms and especially intensive care units because of the very severe patients that required special care," Shindo said, urging hospitals to prepare for the possibility of a significant number of patients requiring intensive care.

"We can expect more severe disease during the upcoming influenza season," she said.

Shindo noted that, although a few cases have been reported of people who have been infected with virus that is resistant to antiviral drugs, the medications remain highly effective for most patients if administered quickly.

"Do not delay treatment," Shindo said. "Do not miss this opportunity for early treatment."

The WHO's warnings came as U.S. health officials announced that the number of states reporting widespread flu had increased from 37 to 41 and regional or local outbreaks were being reported in the remaining parts of the country.

The number of deaths from pneumonia and flu-like illnesses had surpassed what the CDC considers an epidemic level, said Anne Schuchat of the federal Centers for Disease Control and Prevention. About 6 percent of all doctor visits are for flu-like illnesses, she said.

"It's unprecedented for this time of year to see the whole country seeing such high level of activity," she said.