Roche, whose drug Tamiflu is in great demand as a preparation for a possible influenza pandemic, is not the only company reaping a financial windfall from a treatment for a contagious disease. And in this case, the health threat is not merely a potential one. ViroPharma, a formerly struggling biotechnology company, sells Vancocin, the only drug approved to treat Clostridium difficile, a bacterium that already kills thousands of people a year in this country and is apparently becoming more common and more deadly. The life-saving drug has turned out to be a financial lifesaver for ViroPharma which, almost by...