As the heavily populated northern hemisphere edges towards the cooler season, the World Health Organisation (WHO) is urging people around the world to brace themselves for a second wave of the swine flu pandemic, and it might get worse since the virus thrives in winter.
"The WHO is still mobilised and worried," spokesman Gregory Hartl said as the global health watchdog kept an anxious eye on some "mysterious" patterns of illness associated with the new A (H1N1) virus that appeared in April.
WHO Director General Margaret Chan warned on Friday that there had been second and third waves in previous pandemics.
"We cannot say for certain whether the worst is over or the worst is yet to come," Chan said in a videotaped address to a symposium on flu in the Asia-Pacific region.
"We need to be prepared for whatever surprises this capricious new virus delivers next," she added.
Some 1,799 people have died since the A(H1N1) was uncovered in Mexico and the United States nearly six months ago, according to the WHO.
By comparison, an estimated 250,000 to 500,000 people die around the world every year from seasonal flu, and overall the symptoms of the new pandemic virus have proved to be mild in the great majority of known cases.
Swine flu has spread swiftly into 177 countries, proving to be more infectious than seasonal flu and more durable through warmer months. Some 182,000 people worldwide are known to have caught swine flu based on laboratory confirmed cases, but the WHO has long advised countries to give up counting; the true number may in the millions, according to some experts.