Health care just caught a cold.
The Health Care Select Sector SPDR Fund, an exchange-traded fund tracking 62 major health-care companies, has shed more than 2% in the last month. The iShares U.S. Healthcare Providers ETF, a similar fund tracking 47 health-care providers, has lost about 5%.
Uncertainty around the fate of the Affordable Care Act, more commonly known as Obamacare, and rising drug prices, has weighed on the group in recent weeks. The downward moves have left health care as the only market sector still in the red, and the worst-performing one in the last month, after a banner year as 2018's best-performing sector.
Tom Lydon, editor and proprietor of ETFTrends.com, told CNBC's "ETF Edge" on Monday that the weakness could also be tied to a change in investors' priorities.
"In Q4 of last year, we saw big declines in the stock market, but health care buoyed because of [its] value," Lydon said. "Value's a little bit out of favor today, so that's another reason why the big health-care stocks aren't doing as well."
But Dan Wiener, chairman of Adviser Investments, sees something bigger at play.
"You have to remember health care is an enormous part of this economy. We're talking about close to 20% of the economy," Wiener told CNBC in the same "ETF Edge" interview. "You've got the biotechs. You've got the health-care providers. You've got the pharma. You've got the device companies."
Most broad-based health-care ETFs have all of those categories in their holdings, Wiener noted. However, two of those holdings far outweigh the rest.
"[Tuesday] morning, we're going to get [Johnson & Johnson] earnings," Wiener said. "If you look at the big health-care ETFs, what are the two biggest stocks in there? J&J and Pfizer. And we're not talking about 5% positions. Between the two of them, we're talking 15%. Those are big. I mean, if J&J and Pfizer don't have a good day, the whole sector disappears on the ETF side."
That could prove disastrous for the sector's various ETFs, but there's no harm in getting more granular, Lydon said.
"It's kind of been the tale of two cities" with "conventional health-care" companies and the biotechnology names, he noted, with traditional health-care stocks tacking on only mid-single-digit gains year to date, but some biotech stocks up north of 20%.
So, if you're on the hunt for health-care stocks, be sure you know where to look.