A decade of pesky germs, from SARS to avian flu to H1N1, has given rise to dozens of products bragging about their microbe-killing properties. Everything from hand-sanitizing liquids to products like computer keyboards, shopping carts and tissues tout that they kill 99.9%, or 99.99%, of common bacteria and fungi.

But some of these numbers look like the test scores in a class with a very generous grading curve. They often don't include all pesky germs, and are based on laboratory tests that don't represent the imperfections...