KANSAS CITY, Mo. – Researchers have found that domestic ducks seem to act as intermediate hosts for a variety of avian viruses because of their contact with migrating birds.

Ducks, which picked up bird flu from migrating birds, passed the virus on to chickens and humans were likely later infected with H7N9 avian flu after contact with chickens at live poultry markets in China, according to a new study published in the science journal,

A team of researchers set out to trace the root of the outbreak, which has infected 130 people and resulted in 40 deaths. More than 1,300 samples were taken from ducks, geese and other birds in addition to fecal and water samples from live poultry markets. The researchers found that the virus developed in parallel to H7N7, which can infect humans, according to the Nature report. The finding reinforces the idea that H7 viruses are constantly mixing and exchanging genetic material. This mixing broadens the threat that H7N7 will be able to spread to humans.

"The discovery here of a related H7N7 influenza virus in chickens that has the ability to infect mammals experimentally, suggests that H7 viruses may pose threats beyond the current outbreak," the researchers said. "The continuing prevalence of H7 viruses in poultry could lead to the generation of highly pathogenic variants and further sporadic human infections, with a continued risk of the virus acquiring human-to-human transmissibility."

The H7N9 avian flu was so lethal because virus seemed to spread through poultry flocks causing no visible signs of illness in the birds. Therefore, anyone handling live poultry would not be aware that birds are infected.