Scientists and public health officials are concerned about a rapidly evolving health situation in China involving a new avian flu virus, H7N9, that has infected nine people and resulted in three deaths. The genetic sequences of the flu viruses are causing concern, with evidence of the virus adapting to spread in mammals and causing serious illness. The proximity of the infections has raised questions about animal reservoirs and potential person-to-person transmission. The virus is showing signs of evolutionary advantage and genetic stability, posing a potential threat of a new pandemic. Surveillance and investigations are ongoing to understand the virus and its spread, with pandemic readiness activities underway to prepare for a potential outbreak. Public health officials are closely monitoring contacts of known cases, and hope the virus will follow the typical pattern of flu viruses, but so far there is no evidence of this. While pandemic readiness activities are underway and efforts are being made to produce a vaccine seed strain, the unpredictable nature of influenza viruses makes it difficult to predict the outcome of the situation. The WHO and other agencies are working to understand the virus and its potential impact on public health.