Many infectious diseases are becoming harder to cure because they have become resistant to antibiotics. This is what happens: When bacteria infect a person, they constantly multiply, passing on their genetic traits to their offspring. With the production of each new bacterium, there is a chance of a mutation—a slight copying error that will give the new bacterium a new trait. The probability that a bacterium will mutate in a way that makes it able to resist an antibiotic is extremely small. But bacteria reproduce by the billions, sometimes producing three generations of offspring in an hour. Thus, the unlikely does happen—every once in a while, a bacterium occurs that is difficult to kill with an antibiotic.

So when the infected person takes an antibiotic, the nonresistant bacteria are wiped out, and the person probably feels better. However, the resistant bacteria survive. But now they no longer must compete for nutrients and territory with fellow microbes. They are free to reproduce unchecked. Since a single bacterium can multiply into over 16 million bacteria within a single day, it does not take long before the person again becomes ill. Now, however, he or she is infected by a strain of bacteria resistant to the drug that was supposed to kill it. These bacteria can also infect other people and in time mutate again to become resistant to other antibiotics.

States an editorial in the journal Archives of Internal Medicine: "The rapid development of bacterial, viral, fungal, and parasitic resistance to our present therapeutic armamentarium makes one wonder not if, but when we will lose this war of man against the microbial world."—Italics ours.