Countries in the Northern Hemisphere that have already had one wave of swine flu should prepare for a second wave, which may be worse, the agency said.
"The H1N1 pandemic virus has rapidly established itself and is now the dominant influenza strain in most parts of the world," WHO said in a statement. "The pandemic will persist in the coming months as the virus continues to move through susceptible populations."
Clinicians from around the world are reporting a very severe form of the disease in young and otherwise healthy people.
"In these patients, the virus directly infects the lung, causing severe respiratory failure," WHO said.
Therefore, countries should anticipate a growing demand for treatment in intensive care units as they prepare for a second wave of the pandemic, it said.
Flu levels remain elevated in South Africa and Bolivia and many of these cases are probably swine flu, it said. But in most of the Southern Hemisphere, flu levels have returned to normal, said WHO spokesman Gregory Hartl.
At least 209,438 people worldwide have caught swine flu and at least 2,185 died of it, according to WHO. The real caseload is much higher because countries are no longer reporting individual cases.
Hartl said the agency was watching flu rates in Japan, where it believes that the high season for infections is starting earlier than normal.