US stock futures tumbled while safe-haven assets such as the yen and US bonds gained on Friday after media reported that a doctor who returned to New York City from West Africa has tested positive for Ebola.
S&P mini futures fell as much as 0.7 percent to 1,931.75, slipping from two-week highs hit the previous day on budding optimism on corporate earnings and the global economy.
Asian shares also lost some ground with MSCI's broadest index of Asia-Pacific shares outside Japan slipping 0.1 percent.
Japan's Nikkei share average also gave up some of earlier gains, though it was still up 1.0 percent on the day.
"Just when markets got some relief on the world economy, we had this news. Obviously people who have just made bullish bets (on the economy) will close their positions as no one can tell exactly what is going to happen," said the head of currency trading at a Western bank's Tokyo branch.
The first diagnosed case of Ebola in the world's financial hub of New York sent investors rushing to traditional safe-haven assets.
As US bond prices gained, the 10-year U.S. yield fell back to 2.250 percent from Thursday's two-week high of 2.300 percent.
In the currency market, the yen gained 0.3 percent to 108.00 yen to the dollar while the risk-sensitive Australian dollar became the victim of rising caution among investors, falling 0.3 percent to USD 0.8732.
Risk asset prices had risen on Thursday after upbeat US corporate earnings, solid US economic data, and an unexpected uptick in euro zone business sentiment helped ease concerns that the global economy was losing momentum.
Results from Caterpillar Inc and 3M Co reassured investors that companies with large overseas revenue streams could deliver solid profits, despite concerns about global economic growth.
The markets ignored positive earnings earlier this month, when the world's share prices hit multi-month lows, overwhelmed by fears that sluggishness in Europe and Asia could deal a severe blow to the US economy as well.
But a steady flow of solid earnings helped to ease many such concerns for now.
With 177 of the S&P 500 companies having posted third-quarter results, 69.5 percent have beaten expectations, better than the 67 percent beat rate over the past four quarters, and higher than the 20-year average of 63 percent, Thomson Reuters data showed.
"The markets had fallen on sentiment rather than on facts. And the sentiment is coming back," said Soichiro Monji, chief strategist at Daiwa SB Investments.
New claims for US unemployment benefits also held below 300,000 for a sixth straight week last week.
In the euro zone a survey showed businesses performed much better than anyone expected this month, even though it also pointed to strong deflationary pressure in the region.
The euro drew some support from the surprise strength in the euro zone data, but was still near a two-week low of USD 1.2614 hit on Thursday. It last stood at USD 1.2657.
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