Q. Should you avoid contact with a dog that was petted by someone with the flu?

A. The possibility of transmission cannot be ruled out, medical studies suggest, but it would seem to require a perfect storm of bad personal hygiene and other conditions.

Most experts believe that the usual transmission route is by way of minute droplets in the air that are inhaled or swallowed by someone within six feet of the source. Much less often, the next victim may touch a surface with living viruses on it, then touch his or her nose or mouth.

Contact with such surfaces, called fomites, is a far less efficient way to spread the virus than the airborne route, according to an animal study published in The Journal of Infectious Diseases in 2009.

Another study found that the virus could survive for several days on bank notes, but only if there was a large dose of virus to begin with, protected by a considerable amount of respiratory mucus.