Of all the potential global disasters that we periodically list and examine, only one has threatened repeatedly to become a reality: the flu, a variety of which is now spreading quickly across the world. A global flu epidemic seems a feeble threat compared to an asteroid impact or a nuclear holocaust, which nobody has encountered.

Japan confirmed its fourth case of the A(H1N1) flu virus on Sunday in a teenager who returned from a school trip to Canada with the three other Japanese who contracted the ailment, commonly known as swine flu.

Mexico city sprang back to life this week after two weeks of fear and inactivity. Officials shut down most of the economy to halt the spread of a previously unknown strain of the mongrel H1N1 virus, which is comprised of avian, swine and human influenza viruses. The hope is that the outbreak has now peaked.

The World Health Organisation is considering an overhaul of its pandemic rating system amid criticism that it provoked alarm by rapidly escalating its warnings over swine flu.

Officials at the agency's headquarters in Geneva said they were discussing changes to the six-point scale to make clear in the future the gravity of the threat posed by a new virus.

With swine flu now widespread in the United States, the World Health Organization reported Monday that the disease had widened globally, too, with 20 countries reporting 985 laboratory confirmed cases, compared with 18 countries reporting 898 confirmed cases on Sunday.