Editor's note: The following video incorrectly stated that a patient taking Incyte's drug Jakafi passed away from a fatal brain infection. The patient developed PML, but is still alive. The Fool regrets the error.
Shares of biotech Incyte (NASDAQ:INCY) came down hard today, after news that the company's drug Jakafi may have weakened the immune system of one patient in the U.K. enough to allow a rare type of viral brain disease to take hold. In this video, Motley Fool health-care analyst David Williamson gives investors some advice that may temper their panic a bit.