The H5N1 bird flu virus has spread from birds to dairy cattle in the United States where a number of agricultural workers have also been infected by it. This is thought to be the first time humans have caught the virus from another mammal and the first time the virus has been detected in cattle.

This unusual development is being tracked by virologists who have followed Bird Flu since it first emerged in Hong Kong in the 1990s.

Since then, across the world millions of wild birds and poultry have died from the virus and over 400 human deaths worldwide have been linked to it. So it is a concern that the US outbreak has emerged in dairy cattle herds and that there has been some human infection - although there has been no person-to-person infection.

This Inquiry examines how the virus infects birds and mammals and what the potential is for further transmission to humans.

Contributors: Dr Erin Sorrell is a senior scholar and associate professor at Johns Hopkins University in the US. Professor Wendy Barclay studies viruses at Imperial College London in the UK Dr Ed Hutchinson is a virologist at the MRC University of Glasgow Centre for Virus Research in Scotland Dr Marc-Alain Widdowson leads the high threat pathogens group at the World Health Organisation in Europe.

Presenter: Charmaine Cozier Producer: Phil Reevell Researcher: Katie Morgan Editor: Tara McDermott Sound: Nicky Edwards Production co-ordinator: Tim Fernley

(Photo Cows queuing for their midway milking at United Dreams Dairy, in North Freedom, Wisconsin. Credit: The Washington Post/Getty Images