The return of warmer weather to the northern hemisphere is unlikely to bring a let-up in the deadly H5N1 virus because it is already endemic in poultry flocks in several parts of Asia, experts say.

Scientists previously found the bird flu virus to be most active from October to March when temperatures are cooler or below 20 Celsius (68 Fahrenheit), but now they are warning against any complacency with the return of summer.

"I don't think it will go away in the summer months, it will continue to be in poultry," said Hong Kong microbiologist Malik Peiris, who has studied the virus since 1997, when it made its first known jump to humans in Hong Kong, killing six people.

"The virus has been persisting in quite diverse climates, such as Indonesia, where it is hot ... to a large extent it has been maintained in poultry flocks."

The virus resurfaced in late 2003 and has been seen in birds in over 40 countries in Asia, Europe, the Middle East and Africa. It spread quickly in the past two months, infecting 20 new countries.

Although the number of human infections is low at 204 cases, 113 have died and some experts say just a few mutations are needed for the virus to transmit among people, triggering a pandemic.

Outside Asia, human casualties have been most common in Turkey and Egypt, where there have been 24 documented infections and 8 deaths combined -- or mortality rates of about 30 percent, which are lower than rates of 50 percent or more in Asia.