Symptoms of mad cow disease in humans can take more than five years to incubate, and the real risk of the millions of pounds of recalled ground beef consumed over the last two years will be difficult to assess, a Johns Hopkins microbiologist said.

“The risk is very low, but it?s the fact that it?s such a terrible disease and that it?s 100 percent fatal that really scares people,” said Dr. Richard Johnson, professor of neurology and microbiology with Johns Hopkins Medicine.