The team believes intensive chicken farming is helping spread bacteria
Animals are now picking up human diseases, possibly as a result of globalisation, a study suggests.
Researchers from the Roslin Institute in Edinburgh said a strain of bacteria had jumped from humans to chickens.
The team believes the Staphylococcus Aureus bacteria crossed between species 40 years ago, when farming techniques became more intensive.
They argue that fewer breeding lines in an industry dominated by multinationals has helped spread bacteria.
The findings are published in the Proceedings of the National Academy of Sciences.
The researchers believe their discovery is the first clear evidence of bacterial pathogens crossing over from humans to animals and then spreading, since animals were first domesticated 10,000 years ago.
They found that a form of Staphylococcus Aureus - of which MRSA is a sub-type - remained confined to one geographical area in humans, but in chickens it was spread across different continents.
The bacteria are a major cause of animal diseases, including bone infections in poultry
Dr Ross Fitzgerald, of The Roslin Institute, warned that bacteria crossing from humans to animals could have an impact on food security.
He said: ""Half a century ago chickens were reared for their eggs, with meat regarded as a by-product.
"Now the demand for meat has led to a poultry industry dominated by a few multinational companies which supply a limited number of breeding lines to a global market - thereby promoting the spread of the bacteria around the world."