Scientists in Edinburgh say globalisation and industrialisation are causing diseases to spread from humans to animals.
They believe they have uncovered the first clear evidence of humans passing on bugs since the introduction of domesticated animals 10,000 years ago.
Researchers at Edinburgh University's Roslin Institute found a bacteria in chickens which originally comes from humans.
The bug, which is related to the MRSA bacteria, is thought to have crossed between the species around 40 years ago when intensive poultry farming was introduced.
They found the germ was limited to people in one part of the world but had spread to chickens across different continents.
Infectious diseases in chicken flocks can be hugely damaging to the farming industry and the scientists fear that any further spread of disease from humans to livestock could impact on food security.
Dr. Ross Fitzgerald, from The Roslin Institute, 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."
The bacteria are a major cause of animal diseases, including bone infections in poultry.
The scientists now plan to analyse other breeds of livestock to see if they have contracted any human bugs.
