Has swine flu retreated, or is it preparing to return in autumn?

This flu has spread around the world: the World Health Organization had confirmed 4,694 cases in 30 countries by 11 May 2009. But Dr Dickinson of the University of Miami pointed out that for every confirmed case there are hundreds that go undetected. Based on how fast it spread, the WHO estimates there were 32,000 infected people in Mexico alone by the end of April.

There are 2,618 cases and three deaths in the US, which has overtaken Mexico in confirmed cases. Most infections now spread from person to person and have nothing to do with travel to Mexico. So far the illness is mild, and the deaths have involved other hidden health problems. But since most people don't seek treatment, the figures are only a small part of the true number.

Mexico has 2,059 cases, including 56 deaths, which led the country to close government offices, schools and businesses. Schools are now reopening, with teachers reporting any child with symptoms, but sports lessons and ceremonies have been suspended as a precaution.

The death rate from this virus is 4 per 1,000 cases, about as severe as the 1957 Asian flu but milder than the 1918 flu, and it spreads less quickly than the latter. But there are fears that the virus could change, so many think a vaccine is urgently needed: the 1918 virus faded in the summer but came back in the autumn and killed millions.