In this video as part of The Motley Fool's "Ask a Fool" series, Fool health-care analyst David Williamson takes a question from a Fool reader, who asks, "Gilead Sciences (NASDAQ:GILD) seems to be on a down trend. Is there any specific reason, or should I just hold firm?"

David takes a look at Gilead's downward trend year to date but cautions that this 3% pullback since the beginning of 2014 is much better understood in the context of the stock's meteoric rise over the past 12 months. He then discusses for investors the company's hepatitis C drug Sofosbuvir, sold under the name Sovaldi, and how much of the investing thesis is based around the success of this drug. He notes that while the drug is destined to be a blockbuster with an incredible cure rate, it has its risks that investors need to be aware of. Finally, David looks at Gilead's pipeline, and the future of the company.