The new strain of bird flu that killed at least 40 people in China this year likely evolved through close contact between ducks and chickens in markets selling live birds, according to a genetic analysis published in the journal Nature. At least 130 people became infected during the outbreak, which began in March.

“We clearly identified that the source of human infection came from the infected chickens, not any other types of birds,” said Yi Guan, a professor of virology at the University of Hong Kong and one of the study authors. The analysis also shows the virus was shed from the birds' oral or upper respiratory tract, not from fecal material. According to Yi, this suggests that H7N9 reasonably well adapted to infect humans, a finding that's supported by other research.

The research group also discovered a previously unrecognized variety of bird flu – an H7N7 strain, with a genetic makeup similar to the novel H7N9 strain. The H7N7 strain was also able to infect mammals in laboratory conditions.

Any variety of influenza is broadly characterized by two of its proteins: the type of hemaglutinin (“H”) and the type of neuraminidase (“N”).
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