New York (CNN Business) It was shaping up to be a quiet day on Wall Street. Then around midday, stocks sold off sharply and investors scrambled. With no obvious catalyst triggering the selloff, worries about the coronavirus outbreak seemed to be the obvious suspect.

Dow INDU At its worst, thewas off by 388 points, with all three major stock indexes sharply in the red. The drop in stocks followed a dip in Treasury bond yields, which move opposite to prices, and implied that investor appetite for safe haven bonds was high.

S&P 500 SPX Nasdaq Composite COMP Still, investors scrambled to make sense of it. The dramatic selloff only lasted a short while and was followed by a swift rebound. Stocks still closed in the red, however, with the Dow ending 0.4%, or 128 points, lower. Thefell 0.4%, and thefinished down 0.7%.

In recent days, investor confidence built up amid hopes that new cases of the virus were slowing down. But Thursday South Korea confirmed new cases of the virus, hammering home that the outbreak is not over.

The new cases "are leading to concerns that the global supply chain will be impacted in a material way," said Chris Zaccarelli, chief investment officer for Independent Advisor Alliance, in emailed comments.

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