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.

David Williamson has no position in any stocks mentioned. The Motley Fool recommends Gilead Sciences and Quest Diagnostics. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.