Reader's Digest is moving ahead with plans to deliver targeted information and advertising to sufferers of 25 different ailments after receiving overwhelming response to a health survey sent to more than half of its 13.5 million subscribers. The magazine tested its subscribers' interest in receiving information on specific ailments through a questionnaire attached to a sweepstakes form last fall. The questionnaire, which was sent to 7.5 million subscribers, drew an impressive 10 percent response rate and resulted in a total of 5 million information requests. This, coupled with results of a study by Roper Starch Worldwide, New York, in which Reader's Digest subscribers listed the magazine as the second-most trusted source of healthcare information after doctors, convinced officials to build a stronger, targeted healthcare program. Once the results of a questionnaire sent to the balance of its database this month are tabulated, the company expects to amass 10 million information requests. The ailment-specific database is expected to be completed in April. Files are expected to range in size from 100,000 subscribers who are interested in less common ailments such as epilepsy to close to 800,000 subscribers who are interested in information on back pain. The effort marks the first time the Pleasantville, NY, company has segmented its database to specific ailments.