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:

Presenter: Charmaine Cozier

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