* Amazon tumbles, Microsoft climbs after earnings
* New home sales data due (Adds quote, updates prices)
By Chuck Mikolajczak
NEW YORK, Oct 24 (Reuters) - U.S. stocks were set for a slightly lower open on Friday, putting the S&P 500 on track to cut 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 plunged 9.5 percent to $283.40 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, up 4.6 percent at $47.09 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 gained 2.4 percent to $85.22 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 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.
"The market is nervous, people are really paying attention now, you've seen volatility come back big time," said John De Clue, chief investment officer of the Private Client Reserve at U.S. Bank Wealth Management in Minneapolis, Minnesota.
"We were lulled into a sense of complacency and now the market is on high alert, people perceive both threats and opportunities and they are poised to take action pretty quickly."
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.
Futures managed to pare losses in choppy premarket action after the World Health Organization set out plans on Friday for speeding up development and deployment of experimental Ebola vaccines, saying hundreds of thousands of doses should be ready for use in West Africa by the middle of 2015.
S&P 500 e-mini futures were down 2.75 points and fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract, indicated a slightly lower open. Dow Jones industrial average e-mini futures fell 12 points and Nasdaq 100 e-mini futures lost 4.75 points.
Ford Motor Co's 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. (1400 GMT). 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 jumped 9.2 percent to $1.78, hazmat suits maker Lakeland Industries Inc surged 10.8percent to $16.91 and face mask maker Alpha Pro Tech Ltd climbed 17.6 percent to $5.41 before the opening bell.
Editing by Chizu Nomiyama