The surging bull market that started last March — and has pushed the market up an astounding 83% from a year ago — has hit its first serious snag. And many are blaming the highly contagious Delta variant for the selloff.

The first Delta case was logged in the U.S. in March. By mid June, it had become the most common strain. As Fortune’s Erika Fry and Nicolas Rapp reported, according to the CDC, which relies upon genomic surveillance data reported through July 3: “51.7% of new COVID cases are estimated to be caused by the Delta strain of the coronavirus, up from 30.4% for the previous two-week period.”

The stock market, which had been mostly chugging along save for a few dips, began to waver in early July but sustained several days of losses last week. On Friday, CDC Director Rochelle Walensky sounded the alarm about the variant, tweeting: “What’s clear: This is becoming a pandemic of the unvaccinated.” That set the stage for a horrible trading day Monday, with the Dow finishing the day down 726 points or 2.1%, making it the worst trading day of the year. Overall, from its 52-week high of 35,091.56 the Dow is down around 3.2%. The S&P 500 is down 3.1% from its July high of 4,393.68.

That said, those highs were putting stocks at valuations that were hard to fathom. As Fortune‘s Shawn Tully wrote last week, “A highly respected valuation gauge developed by Yale economist and Nobel laureate Robert Shiller hit a mark showing that the S&P 500 is now pricier than in 96% of all quarters over the past 141 years. Put differently, big-cap stocks have been this expensive only 4% of the time in the recorded history of equity markets.”

Whether this patch marks a cooling bull market or merely an inevitable dip on a continued upward trajectory remains to be seen. As LPL Chief Equity Strategist Jeff Buchbinder told Fortune, “With no 5% pullbacks for the S&P 500 since last October, we are due for some volatility, especially considering the S&P 500 is up more than 90% from the March 2020 lows.”

As for which sectors fared the best and worst? Investors bailed out of leisure and travel stocks (think airlines, hotels, cruise stocks) and rotated into some familiar pandemic names such as Peloton and Moderna, which got a boost on news that it is joining the S&P 500. And yes, there was one final name that could not escape Delta’s wrath on Monday: Delta Airlines was down 4%.