Stocks have kicked off the new week on a downswing, as the Dow Jones Industrial Average (^DJI 0.06%) remained 18 in the red in as of 2:30 p.m. EDT. While the index's stocks are leaning a bit toward the laggards, a bloodbath in the health-care sector has taken its toll on the blue-chip index. Merck (MRK -0.11%) and Pfizer (PFE -0.12%) both have hit big stumbling blocks today as part of the sector's downturn. Let's catch up on what you need to know.

Manufacturing mixed, but health care dives
A pair of flash purchasing managers' indexes disappointed investors to start the morning morning. The Markit preliminary PMI for March fell by more than 1.5 percentage points to a mark of 55.5 from February's 57.1. Still, that's strong expansionary territory for the PMI if it holds up through the month's final reading, giving U.S. manufacturing plenty of space ahead of any sign of contraction at the 50-point mark.

Things aren't so rosy across the Pacific in China, where the world's second-largest economy saw its own flash PMI fall by 0.4 percentage points to 48.1 in March, down from February's already-worrying mark of 48.5 . China's manufacturing sector hasn't been able to dig itself out of contraction in 2014. With concerns mounting over the nation's ability to maintain sinking annual economic growth targets, it's worth keeping an eye on whether Beijing will toss its sluggish manufacturing a lifeline in the form of stimulus. For China investors who have weathered a bloodbath in Chinese stocks over the past year, that would come as a welcome respite after quarters of disappointment.

Still, today's biggest market shake-up has come from health care, as major stocks across the sector plunged into the red. The Dow's Merck and Pfizer couldn't avoid the hit, with these two Big Pharma stocks sinking 1.7% and 2%, respectively, so far. Part of the blame can be pinned on investors taking gains after what's been a stellar intro to 2014 for the sector. Merck has racked up a 10% gain to take the Dow's top performer spot so far this year, while Pfizer is also among the Dow's five best year-to-date stocks.

Today's pullback shouldn't give you cause for concern. While Merck and Pfizer have struggled with falling sales due to losses of top drug patents lately, both companies have managed to turn the corner toward brighter futures. Pfizer looks among the best-positioned stocks in the sector, witih a whopping 26 drug programs in phase 3 trials or later as of last November. Pfizer's current portfolio has continued to excel, as top seller Lyrica managed 11% revenue growth in 2013.

Merck's future is a little hazier after years of sluggishness and disappointment out of the company's research and development front. All eyes are on the company's developmental cancer therapy MK-3475, which could one day bring in billions of dollars in annual revenue if it gains multiple approved indications, With Merck's pipeline otherwise somewhat bare, shareholders need to fix both eyes on this drug's progress in the coming year.