The E.coli bug
Sheep carry a virus which could be harnessed to kill the E.coli food poisoning bug.
The discovery could help eradicate E.coli 0157 in farm animals, reducing the chance of humans becoming infected with the bug through the food chain.
Three-quarters of cases can be traced directly back to livestock, which can harbour the infection without becoming ill.
Meat can become contaminated when the animals are slaughtered. Manure can also be a source of infection.
Researchers from Evergreen State College in Washington state were testing antibiotics to see which could eliminate E.coli from animals.
The first stage was to infect sheep with E.coli. But the bacteria kept disappearing.
The animals were re-infected three times, but each time, the bacteria vanished.
Scientists discovered that the sheep carried a bacteria-killing virus, or bacteriaphage, that infects certain E.coli strains.
Laboratory tests showed the CEV1 phage killed 16 out of 18 toxic strains of E.coli, but only eight out of 73 harmless strains.
Further tests in sheep showed the bacteriaphage cut the number of E.coli present by 99% in two days.
Dr Andrew Brabben, who led the research, hopes his findings will be help wipe out E.coli in herds and flocks.
He said phages, unlike antibiotics, only attack particular bacteria.
In addition, antibiotics are expensive and need to be given to every animal, while only one would need to be infected with CEV1 for it to be passed to a whole herd.
The phage also appears to continue to reproduce itself after all the toxic E.coli has been destroyed by replicating itself in a harmless E.coli strain.
Dr Brabben said it was more practical to give the phage to animals rather than humans.
Destroying E.coli 0157 can release large amounts of its toxin, which can make a patient's condition worse.
In addition, animal treatments do not have to meet the same safety standards as in humans.
However, the researchers will have to show that treating animals with the phage will not affect the delicate balance of flora in human guts if they are passed on via the food chain.
The research was presented to the Society for General Microbiology meeting and is published in New Scientist magazine.