The two most feared germs are bacteria and viruses. During the 1950s microbiologists made remarkable advances in the understanding and control of these tiny enemies. But not all viruses and bacteria are harmful; on the contrary, many forms pose no threat to humans. In fact, the body depends on good bacteria to function properly.

Differences.

The important difference between bacteria and viruses for the nonscientist is that while bacteria can be killed, viruses can sometimes only be disabled. Though bacteria are larger than viruses, both are invisible except through microscopes. One gram (one-twenty eighth of an ounce) of dirt contains about one hundred million bacteria, and a million viruses can fit onto the head of a pin. Both cause disease by invading cells and either killing them or disrupting their processes. Bacteria reproduce quickly, especially under unsanitary conditions. Viruses are mysterious parasites that...