We turn now to the growing concerns over the spread of bird flu. A third human case has now been reported in the US, another farm worker in Michigan. While officials say the risk to the general public remains low, work on vaccines is already underway. Here's ABC's Phil Lipoff.

Tonight, scientists are ramping up vaccine testing to prevent bird flu transmission between cows and people, as the CDC confirms a third rare human case in the US. A cow likely infecting a Michigan dairy worker, now the first to show respiratory symptoms like coughing. That patient now on an antiviral medication and recovering at home. The previous two cases showed symptoms like pink eye. Federal health authorities investing millions to limit the spread among livestock. The USDA announcing an additional $800 million in emergency aid for testing, virus surveillance, developing bird flu vaccine for livestock, and food safety studies. Federal officials still requiring testing before cattle cross state lines. The USDA confirming 68 outbreaks in dairy cow herds across at least nine states since March. The USDA says, Ultimately, we want to eliminate the virus in developing a vaccine to prevent another emergence of H5N1 in cattle will be an important step toward that end. But that step in humans is likely still far off.

Ultimately, if we reduce the virus amounts in livestock, it's going to reduce the chance that this virus has for jumping into humans.

Well, there is no science to suggest that bird flu is being transmitted from human to human. Scientists are still testing vaccines in humans. Those at highest risk farm workers with exposure to livestock.