Concerns have been raised in recent months after an outbreak of avian flu caused by the virus H5N1 was detected in cattle in the US. To date, 139 affected herds have been identified, and four dairy workers have contracted the virus. The UK Health Security Agency, which previously believed there to be minimal risk of the virus evolving into a form which could spread among humans, now believes there is up to a one-in-three chance of it doing so.