A virus that causes respiratory infections, especially in young children, started out in birds, researchers say.

The bug crossed over to humans about 200 years ago, they said.

The author of the study said in a news release that human metapneumovirus, known as HMPV, may be the second most-common cause of infections in young children. By the age of 5, virtually all children have been exposed. It is also related to RSV, measles and mumps.

Dr. Ron Fouchier of ErasmusMC in the Netherlands said that the virus is closely related to one in birds, AMPV-C, and tracing the quick evolution could help scientists monitor emerging viruses.

"Besides the evolutionary history of metapneumoviruses, we also investigated the mutation rates and the selection pressures of these viruses. An understanding of how viruses evolve and how they adapt to new hosts and their immune systems is important, especially if we are to prepare for new, potentially pandemic diseases," he said.

The information was published in the December issue of the Journal of General Virology.