The Federal Trade Commission has notified hundreds of Web site and newsgroup operators that they must have proof to support their medical- and health-related claims. The FTC, along with U.S., Canadian, and Mexican public agencies, found an abundance of ads and information targeted at sufferers of arthritis, diabetes, and AIDS, for example. In a few weeks, the FTC will check the sites for changes and could file federal charges if inaccurate information remains posted.