"And for mutation to an independent form capable of direct human-to human transmission?"

Only one case of human to human transmission has been reported.

To date, no known cases of dog to human transmission have occurred in the United States.

"They differ technically in several ways from any known human transmissions."

However, there have been a few cases in which this virus underwent human to human transmission.

H5N1 infections in humans are generally caused by bird to human transmission of the virus.

"It really had to be human transmission, because there were no other cases in the village," Dowell told me.

We don't really know the correlation between finding infected birds in an area and relative risk to human transmission.

Human to human transmission has occurred, but it is even more uncommon.

A few isolated cases of suspected human to human transmission exist.