(Bloomberg Business) -- Please open yourstock-market playbook to the chapter on defensive industries, andflip to the page about health-care stocks. Now, rip that page outand throw it away.

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The old playbook has always treated stocks of drugmakers andhealth insurers as the place to turn when times get tough. The ideais that, theoretically, people will continue to buy the medicinethey need long after they've stopped splurging on new clothes or anew car or even a night out on the town.

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As a result, health-care stocks tended to perform better thanother industries during bear markets. But a funny thing happenedover the last few years of the latest bull run: health-care stocksoutperformed by a long shot, including a market-leading advance of12 percent so far this year.

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The rally has lifted the market value of health-care stocks inthe Standard & Poor's 500 Index alone to almost $3 trillion, or15 percent of its entire market cap. The only time it was higher inthe past two decades was during the bear markets following thedot-com bust and the 2008 financial crisis.

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This time, however, investors are playing offense rather thandefense. Love it or hate it, the Affordable Care Act has createdmillions of more customers for health companies.

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Read: Hospitals rally as Supreme Court upholdsPPACA subsidies

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And the booming optimism over biotechnology has sent drugmakerslike Celgene Corp., Endo International Plc, RegeneronPharmaceuticals Inc., Gilead Sciences Inc. and VertexPharmaceuticals Inc. up more than 200 percent since the beginningof 2013, while merger frenzy among managed-care companies hashelped fuel the most recent phase of the rally.

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Read: PPACA pays off for carriers

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The enthusiasm has turned health-care stocks into themost-expensive among the 10 main industries in the S&P500.

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The group's price-to-earnings ratio is more than 24, its highestsince the bear market at the turn of the century, and all but ahandful of the companies trade at a valuation that's above theS&P 500 as a whole.

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Health-care and consumer-discretionary stocks are the onlygroups among 10 forecast to post full-year earnings growth above 10percent for this year through 2017, according to Bloomberg's tallyof analysts' estimates.

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So not only has the page from the playbook been ripped out, it'spractically been doused with gasoline and set on fire. Is it timeto be worried? Maybe, maybe not.

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It is, at the least, time to gather in a BobNewhart-stylegroup-therapy circle and share our anxieties about thecurrent leaders of the stock market, which is sort of what somestrategists are doing these days.

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"What happens when health care stops being such a strongperformer or simply hits a valuation or fundamental pothole?"ConvergEx Group's Nicholas Colas wrote in a note this morning."The simple answer is that other sectors have to step in withbetter performance, or the U.S. equity market willstumble."

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In Colas's estimation, only financial and technology stocks havethe weight and earnings prospects to keep the stock-market partygoing.

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"Should investors worry that the health care sector is gainingweight?" Sam Stovall at S&P Capital IQ asked in a reportyesterday. "We still recommend an overweighting to health care as aresult of unrealized growth from the ACA. We believe this increasein the insured population should be the main driver of top- andbottom-line growth."

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In other words: Thanks, Obama!

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