What happened

Healthcare stocks of all shapes and sizes felt the wrath of the moody market last month. For example, shares of Exact Sciences (NASDAQ:EXAS), a rapidly growing molecular diagnostic company, fell by a jaw-dropping 19.1% over the course of December, according to data from S&P Global Market Intelligence. In fact, Exact's stock was among the worst performers across the entire healthcare sector last month.

Check out the latest Exact Sciences earnings call transcript.

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So what

Why was Exact's stock in particular hit hard by this marketwide pullback? While there is no clear answer -- Exact didn't release much in the way of news in December -- the company's stately valuation may offer some clues. In short, Exact has long garnered one of the richest valuations in the healthcare sector, thanks to the promise of its novel colon-cancer test Cologuard. So it's plausible that the market simply decided to reevaluate Exact's lofty valuation, while it went through a pronounced de-risking cycle last month. 

Now what

Exact's shares, though, have started to rebound in a big way early in 2019. Over the first two weeks of trading, for instance, the stock has already jumped by a noteworthy 16.2%. What's sparking this new year's rally? Exact's shares appear to be heating up in response to the broader market's sudden change of course, as well as the company's strong preliminary fourth-quarter results released last week.

Keeping with this theme, Exact's partnership with Pfizer is reportedly helping to drive an appreciable surge in Cologuard's sales -- even though this deal wasn't inked until late August and has yet to be fully implemented. If Cologuard's commercial trajectory continues to accelerate as a result of this co-marketing campaign, Exact's shares should have little trouble reclaiming their lost ground -- perhaps making this mid-cap biotech stock a worthwhile buy this month.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.