A new strain of the MRSA "superbug" has been found in British cows and is believed to be infecting humans.
Environmental campaigners say the new strain has emerged because of the over-use of antibiotics by dairy farmers, the BBC reports.
Dr Mark Holmes of Cambridge University, who led the research, said this was a "credible hypothesis".
However, researchers writing in The Lancet Infectious Diseases journal say there is no additional health risk from eating milk and dairy products.
MRSA, or methicillin-resistant Staphylococcus aureus, is a drug-resistant form of a usually harmless bacterium that can be deadly when it infects wounds.