U.S. stock index futures fell on Friday, putting the S&P 500 on track to trim its weekly advance, following disappointing earnings from Amazon and as the first diagnosed case of Ebola in New York City raised concerns about the spread of the virus.

Amazon (AMZN.O) plunged 9.9 percent to $282.23 in premarket trade after the online retailer's sales projections for the crucial holiday quarter disappointed Wall Street and third-quarter results missed forecasts.

Microsoft (MSFT.O), up 2.6 percent at $46.17 before the opening bell, may help curb declines after it reported higher-than-expected quarterly revenue while keeping its profit margins largely intact.

Procter & Gamble (PG.N) gained 2 percent to $84.90 in premarket after the world's largest household products maker posted quarterly results and said it would split its Duracell battery business into a separate company.

The S&P 500 .SPX is up 3.4 percent for the week, putting the index on pace for its first weekly gain in five, powered largely by solid corporate earnings reports. The benchmark index is down 3 percent from its record high on Sept. 18.

Concerns over the spread of Ebola continued to vex the market. A doctor who worked in West Africa with Ebola patients was in an isolation unit in New York City on Friday after testing positive for the virus, becoming the fourth person diagnosed with the disease in the United States and the first in its largest city.

Ford Motor Co's (F.N) third-quarter earnings beat Wall Street expectations on a strong showing in North America and China. Shares of the automaker edged up 0.4 percent to $14.45 in premarket trade.

New home sales data for September is due at 10:00 a.m. Expectations call for a seasonally adjusted annual rate of 470,000 units, down from the 504,000 in the prior month.

Ebola-related shares advanced over worries about a possible spread of the virus. Biotechnology company Ibio Inc (IBIO.A) jumped 14.1 percent to $1.86, hazmat suits maker Lakeland Industries Inc (LAKE.O) surged 19.6 percent to $18.25 and face mask maker Alpha Pro Tech Ltd (APT.A) climbed 21.5 percent to $5.59 before the opening bell.

Futures snapshot at 8:10:

* S&P 500 e-minis ESc1 were falling 7.25 points, or 0.37 percent, with 233,855 contracts changing hands.

* Nasdaq 100 e-minis NQc1 were down 16.5 points, or 0.41 percent, in volume of 37,181 contracts.

* Dow e-minis 1YMc1 were down 44 points, or 0.26 percent, with 39,600 contracts changing hands.