US stocks wrapped up their best quarter in more than two decades on Tuesday, bouncing back from historic losses in the first three months of 2020.

The Dow had its best three months since the first quarter of 1987, climbing 17.8%. The S&P 500 gained 19.9%, its biggest quarterly advance since 1998. The Nasdaq did even better, adding 30.6% in its largest upward swing since 1999.

The stunning rebound has been fueled by trillions of dollars in stimulus from Congress and the US Federal Reserve, which is responding to the recession caused by the coronavirus pandemic with unprecedented levels of support.

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And of course, there is the matter of the pandemic. Dr. Anthony Fauci, the top US infectious disease expert, told lawmakers on Tuesday that he wouldn’t be surprised to see the number of new coronavirus cases spike to 100,000 a day.

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“We are now having 40-plus thousand new cases a day. I would not be surprised if we go up to 100,000 a day if this does not turn around and so I am very concerned,” he said.

Investors know the economy is in very bad shape. But many are hoping for a rebound in the second half of the year as lockdowns ease and people get back to work. That process could be slower than expected.

“There’s enough pent-up economic demand to keep Q3 a positive one for economies and risk assets,” said Deutsche Bank analyst Jim Reid.

But the final three months of the year could be rougher as social distancing prevents a full return of normal economic activity and the onset of winter in the northern hemisphere encourages more indoor life and caution, Reid said.

“So at this stage we think you’ll see a reversal of Q3’s gains,” he said.

What can be done to prevent a reversal in stocks and the economy?

Something simple: Fauci expressed dismay over people congregating in crowds, not wearing masks and paying inadequate attention to guidelines on reopening. “We’re going to continue to be in a lot of trouble, and there’s going to be a lot of hurt if that does not stop,” he said.

Goldman Sachs, for one, is making the case for a national mask mandate, saying widespread adoption could save the US economy.

Something complicated: Treasury Secretary Steven Mnuchin and Federal Reserve Chairman Jerome Powell pledged on Tuesday to consider additional stimulus to help speed the economic recovery.

But there weren’t many details on what a potential deal between the White House and Democrats would look like. House Democrats have proposed a $3 trillion relief plan, while Republicans have a smaller package in mind.

Meanwhile, the Fed continues to track the pandemic.

“Hypothetically, a second outbreak would force governments and people to withdraw again from economic activity,” Powell said. “The worst part of it would be to undermine public confidence, which is what we need to get back to lots of kinds of economic activity that involves crowds.”

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