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The spread of the coronavirus across national boundaries—and the lack of predictability in reported numbers—is spreading panic, at least among investors.

On Thursday all major indices took a brutal beating: the smallest loss was in the Russell 2000, down 54.88 (3.5%). But other U.S. indexes took much bigger hits: S&P 500 down 4.4%, Dow Jones Industrials down 4.4%, and the Nasdaq was down 4.6%.

And a big part of the floor falling away happened in the last few minutes. “I can’t remember when it’s been down this much so fast,” said Susan Schmidt, head of U.S. equities at Aviva. “We’re scratching our heads.”

“In true disclosure, I was one of the adults saying at midday everything is going to be alright,” said Lisa Shalett, chief investment officer of Morgan Stanley Wealth Management. “But when I saw 1100 down on the Dow, I recognized that was not the right assessment.”

The markets also saw more than just a one-day thrashing. All the indexes are now deeply red year-to-date: 9.7% for the Dow and Russell 2000, 7.8% for the S&P 500, and Nasdaq at 4.5%.

Losses for the S&P 500 and Dow since last Friday are the worst the market has seen since 2008. For the Nasdaq it’s necessary to go back to 1998.

S&P sector losses for the year have varied wildly: while utilities are off by 1.1%, energy is down 25.7%.

The energy sector loss is due to reduced use of oil in China as well as people putting off travel. Materials feel the pinch because manufacturers face supply chain outages. And financials because they will be hit by widespread economic weakness.

Given that an epidemic was the trigger, it could have been much worse, according to Nancy Perez, a managing director at Boston Private Bank & Trust. “When you look at the avian flu in 2006, the market probably declined about 35%,” she said, especially when the June 2006 outbreak was followed by one of dengue fever in September of that year, according to data from First Trust. Within six months, however, the S&P was already back up 11.7% and had gained 18.4% within a year.

Trouble also had probably been brewing already. “Market sentiment is coming off a 20-year extreme from just a couple weeks ago,” Doug Ramsey, a chief investment officer at the Leuthold Group said in a message to Fortune. “Based on that alone, we would’ve expected a correction of at least 6-8%.” That leaves Ramsey worried that markets may then not have fully priced in the potential COVID-19 economic impact, or the fall could have been steeper.

“The difference in this [epidemic] is the economy was already slowing,” Perez said. Markets expected assistance from trade deals kicking in, a potential further drop in interest rates, and an improvement in the EU’s economic situation.

“We have over the last six months been quite cautious about where we are,” Shalett said. “We have taken a position, up until today … that this is a market that was bound to crack and crack pretty hard. The coronavirus was just the catalyst to let the genie out of the bottle.

At this point, the only certainty is uncertainty, and that’s bad for equities.

“Stock prices are based on expectations, not realizations, and what we have is that some investors are pricing in a true global pandemic, in essence, assuming a worst-case scenario,” said Robert Johnson, professor of finance at Creighton University. “There is a flight to quality and bond yields are dropping precipitously. Ironically, those investors piling into treasuries are going into the most overvalued asset.”

And for better or worse, there’s still another trading day left this week.