Health care investors have grown accustomed to four days of optimism when Wall Street decamps to San Francisco each January to hear about science, deals and fresh opportunities at the J.P. Morgan Health Care Conference.

Instead, on Monday, it was practically a bloodbath. The 190-member Nasdaq Biotechnology Index fell 3.4 percent, the worst opening day of trading during the conference since 2001, when the broader market was in a downturn. The 56-member Standard & Poor’s Health Care Index fell 1.2 percent, its worst conference Monday since 2009. Stocks plunged, taking down everything from tiny biotechs to large cap drugmakers.

“The stock market has been brutal this year so far,” said BioMarin Pharmaceutical Inc. Chief Executive Officer Jean-Jacques Bienaime. “We’ve had a tough start.”

While BioMarin’s shares are down 11 percent since the start of the year, others had it that bad or worse on Monday alone. Bluebird Bio Inc., which is developing cancer treatments and gene therapies, fell 19 percent. Spark Therapeutics Inc., one of biotech’s darlings for its potentially curative therapy for certain forms of blindness, fell 12 percent. The biotech index closed at its lowest price since Sept. 2015.

“This is just a year where everyone looks at the glass as half empty,” said Sven Borho, a partner at the investment firm OrbiMed Advisers. He blamed the dark mood of investors, not concrete bad news. “You are looking for fundamental explanations for the market weakness -- you will not find it.”

That wasn’t entirely true. McKesson Inc. fell 10 percent after the drug and health products distributor said that it would be hurt by a slowdown in the growth of generic drug prices. Celgene Corp. dropped 5.5 percent after reporting preliminary fourth-quarter earnings results below analysts’ estimates, and naming a new CEO. And on Friday, Humana Inc. said it would take a loss on health insurance plans sold through Obamacare. Even Medtronic Plc, which announced a $5 billion share buyback and raised the low end of its fiscal 2016 forecast, fell 1.1 percent.

“It’s bad, sad, unhappy,” Les Funtleyder, a health-care portfolio manager with E Squared Asset Management, in a brief interview in a stairway of the Westin St. Francis hotel, the conference’s home. Turmoil in Chinese stock markets and oil had already rattled the markets, he said. “On balance we had a weak mood coming in, and now you have fundamental evidence that it’s bad. Negative sentiment is becoming tangible news.”

Borho called some of those selloffs overdone, and asked why investors weren’t reacting more strongly to positive news announced Monday in San Francisco, like Illumina Inc., which issued sales and earnings estimates above what analysts had forecast. Hospitals, led by HCA Holdings Inc., also gained Monday -- after large declines in the last half of 2015.

And in the long run, it’s still been a very good few years for health investors. Over the last five years, the Nasdaq Biotechnology Index has more than tripled returns in the S&P 500, and the S&P Health Index has almost doubled the broader basket of stocks.

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