Some lovers' presents we cherish—the romantic dinner, the cashmere sweater, the love note. Sexually transmitted diseases (STDs), however, are the mementos we all could do without. Unfortunately, STDs tend to be the gifts that keep on giving as people—often unknowingly—pass them on from person to person. "More than fifteen million new infections occur each year," says Linda Alexander, Ph.D., president and CEO of the American Social Health Association. "But STDs are silent infections. You may not have visible symptoms—especially if you're a woman."…