WASHINGTON (Reuters) - Doctors are reporting a severe form of swine flu that goes straight to the lungs, causing severe illness in otherwise healthy young people and requiring expensive hospital treatment, the World Health Organisation said on Friday.

Some countries are reporting that as many as 15 percent of patients infected with the new H1N1 pandemic virus need hospital care, further straining already overburdened healthcare systems, WHO said in an update on the pandemic.

"During the winter season in the southern hemisphere, several countries have viewed the need for intensive care as the greatest burden on health services," it 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."

Earlier, WHO reported that H1N1 had reached epidemic levels in Japan, signalling an early start to what may be a long influenza season this year, and that it was also worsening in tropical regions.

"Perhaps most significantly, clinicians from around the world are reporting a very severe form of disease, also in young and otherwise healthy people, which is rarely seen during seasonal influenza infections," WHO said.

"In these patients, the virus directly infects the lung, causing severe respiratory failure. Saving these lives depends on highly specialized and demanding care in intensive care units, usually with long and costly stays."

MINORITIES AT RISK

Minority groups and indigenous populations may also have a higher risk of being severely ill with H1N1.

"In some studies, the risk in these groups is four to five times higher than in the general population," WHO said.

"Although the reasons are not fully understood, possible explanations include lower standards of living and poor overall health status, including a high prevalence of conditions such as asthma, diabetes and hypertension."

WHO said it was advising countries in the Northern Hemisphere to prepare for a second wave of pandemic spread. "Countries with tropical climates, where the pandemic virus arrived later than elsewhere, also need to prepare for an increasing number of cases," it said.

Every year, seasonal flu infects between 5 percent and 20 percent of a given population and kills between 250,000 and 500,000 people globally. Because hardly anyone has immunity to the new H1N1 virus, experts believe it will infect far more people than usual, as much as a third of the population.

It also disproportionately affects younger people, unlike seasonal flu which mainly burdens the elderly, and thus may cause more severe illness and deaths among young adults and children than seasonal flu does.

"Data continue to show that certain medical conditions increase the risk of severe and fatal illness. These include respiratory disease, notably asthma, cardiovascular disease, diabetes and immunosuppression," WHO said.

"When anticipating the impact of the pandemic as more people become infected, health officials need to be aware that many of these predisposing conditions have become much more widespread in recent decades, thus increasing the pool of vulnerable people."

WHO estimates that more than 230 million people globally have asthma, and more than 220 million have diabetes. Obesity may also worsen the risk of severe infection, WHO said.

The good news -- people infected with AIDS virus do not seem to be at special risk from H1N1, WHO said.

(Editing by Mohammad Zargham)