'Vaccines aren't supposed to cause disease. But that appears to be what's happening on Australian farms. Scientists have found that two virus strains used to vaccinate chickens there may have recombined to form a virus that is sickening and killing the animals.

"This shows that recombination of such strains can happen and people need to think about it," said Glenn Browning, a veterinary microbiologist at the University of Melbourne, Parkville, in Australia and one of the co-authors on the paper.'

Read more: Vaccines Combine to Produce Deadly Virus