New data from Mexico and case numbers so far suggest that if the spread of H1N1 "swine flu" continues elsewhere as it has in the Americas, the virus could infect more than a billion people by July.
The data also suggests that summer temperatures in temperate countries may not slow the virus. However, it spreads slowly enough to respond to the "social distancing" measures used in Mexico.
H1N1 has been circulating, geneticists estimate, since last autumn, but it was first recognised in Mexico in April, New Scientist reports.
New data released by the Mexican health ministry (pdf) reveals disturbing similarities with the last H1N1 pandemic, in 1918.
Health officials have expressed hopes that summer weather in the northern hemisphere will stop H1N1, as it does ordinary flu. But "pandemic flu doesn't seem to be as sensitive to warm weather," says Lone Simonsen of George Washington University in Washington, DC.
A relatively mild first wave of the 1918 pandemic spread through the northern hemisphere in the spring and summer.