Since it first appeared in Hong Kong in 1997, the H5N1 avian flu virus has been slowly evolving into a pathogen better equipped to infect humans. The final form of the virus, biomedical researchers fear, will be a highly pathogenic strain of influenza that spreads easily among humans.

In this week's BMJ, a senior doctor advises caution over the use of the antiviral drug oseltamivir (Tamiflu).