Before a mysterious respiratory illness emerged in the center of China, spreading with lethal effect through the world’s most populous nation, concerns about the health of the global economy had been easing, replaced by a measure of optimism.

The United States and China had achieved a tenuous pause in a trade war that had damaged both sides. The specter of open hostilities between the United States and Iran had reverted to stalemate. Though Europe remained stagnant, Germany — the Continent’s largest economy — had escaped the threat of recession.

Now, the world is worrying anew.

An outbreak originating in China and reaching beyond its borders has summoned fresh fears, sending markets into a wealth-destroying tailspin. It has provoked alarm that the world economy may be in for another shock, offsetting the benefits of the trade truce and the geopolitical easing, and providing new reason for businesses and households to hunker down.