Fearing that many countries would be ill prepared to deal with this potential threat, the World Health Organization in December urged all countries to develop or update their pandemic strategies. The UK government is currently revising its plans.

To date, 12 people have died of the infection in Thailand. But two people there who died of the infection apparently had no direct exposure to birds, suggesting they got the virus from another person, say researchers in an “early release” article in the New England Journal of Medicine (www.nejm.org, 24 Jan 2005). The cluster started with an 11 year old girl who played and slept near infected chickens. Although there have been other instances in which doctors suspected that bird flu had spread between humans, it was always hard to be sure if the victims had not just been exposed to the same source of the virus.

In this case, however, the evidence suggests that the 11 year old Thai girl transmitted the disease to her mother and aunt.

Doctors investigating the deaths talked to the girl's surviving family and healthcare workers. They also tested samples from the aunt and the dead mother and the girl. The mother, a garment worker, had not been around poultry. She was in the girl's house for only 10 minutes. The aunt had had no exposure to poultry for 17 days before falling ill. That is longer than the typical two to 10 days before symptoms usually appear after infection with this virus.

Neither the mother nor the aunt spread the disease to anyone else, an indication that the virus still cannot spread efficiently among humans, the researchers reported. Laboratory tests showed that the virus that infected the family had not mutated from its avian form, the researchers said.

“There is so much transmission going on between birds and humans [in Asia] that the likelihood of a genetic reassortment that would make the virus able to be transmitted in humans grows every day,” says Dr Michael Osterholm, an infectious disease epidemiologist at the University of Minnesota.