Some 2 percent of infections, Dr. Chan said, have resulted in severe illness, with rapid progression to pneumonia. Based on preliminary data, asthma, cardiovascular disease, autoimmune disorders and obesity are the underlying conditions that most put people at greater risk. Around a third to a half of the severe cases occurred in previously healthy young people, she said.

Pregnant woman are also at heightened risk, a particular concern for the developing world, Dr. Chan said, which already reports 99 percent of maternal childbirth deaths worldwide.

Typically, pandemics take six to nine months to move throughout the world. This one has been detected early, giving health officials what Dr. Chan called a “head start.” And as the disease continues, it will be important to add to the health resources of poorer countries, Dr. Chan said. Vigilance must be maintained, she said, “for the next year or two  and beyond.”

“Influenza pandemics, whether moderate or severe, are remarkable events because of the almost universal susceptibility of the world’s population to infection,” Dr. Chan said. “We are all in this together and we will all get through this together.”

The W.H.O. released a report on Wednesday saying that 74 countries had reported 27,737 cases of the disease and 141 deaths since April. Those cases had been heavily concentrated in the Americas, but the rise in cases in Australia and elsewhere indicate communitywide spread in other world regions. According to W.H.O. rules, the organization should declare a pandemic once it finds evidence of widespread “community transmission”  meaning beyond travelers, schools and immediate contacts  on two continents.

The last pandemic, the Hong Kong flu of 1968, killed about 700,000 people worldwide. Ordinary flu kills 250,000 to 500,000 people each year, international health officials have said.

Meanwhile, around the world, efforts to limit the spread of the flu continue. In Hong Kong, which is especially skittish about the flu after its experience with a lethal SARS outbreak in 2003, authorities ordered all kindergartens, primary schools and day care centers to close after an outbreak of swine flu was reported at a local secondary school. The order, effective Friday, will last at least two weeks and affect about a half million students.