The detection of bird flu virus this week in a batch of unpasteurized milk sold in California underscores what health experts say are the known and potential risks of the products.

Scientists are warning about the continued transmission of dangerous H5N1 bird flu, saying that the virus is more widespread than believed.

In the 2009 swine flu pandemic, multiple reassortment events in pigs and birds led to the novel H1N1 virus strain, which led to 60 million cases and 12,000 deaths in the U.S. in its first year of circulating, per the CDC.