Even single-celled organisms desire partners every now and then. Leishmania—single-celled parasites that cause infections of the skin and internal organs—have long been known to multiply asexually, like bacteria. But occasionally, researchers have found hybrid parasites that carry genetic material from more than one strain—or even more than one species—of Leishmania, suggesting that some kind of genetic mixing is going on.Now, researchers at Washington University School of Medicine in St. Louis and the National Institutes of Health (NIH) have found that the hybrid Leishmania parasites can mate with one another to produce fertile offspring that carry genes from both parents—signs of...