Each new drug works for a few years, until a resistant strain of malaria emerges. The World Health Organization estimated that in 1996 there were 8 million new cases of tuberculosis and 3 million deaths from the disease. So variable is susceptibility to TB, however, that 90 percent of those infected never become ill. Like many people my age, who grew up before the era of antitubercular drugs, I have a positive skin test indicating prior infection, but I never had tubercular symptoms or an abnormal chest X-ray. Untold quadrillions of tubercle bacilli live cozily with their human hosts, without biting the lungs that feed them.
Mycobacterium tuberculosis consists of many different strains which are evolving rapidly.
Some varieties are more infectious than others. The more lethal types have an affinity for people with AIDS, whose immune system is compromised by the HIV virus. During the past year, a new strain was discovered in AIDS patients that is resistant to all three standard antitubercular drugs. As diseases evolve in the complex struggle for survival, so do people. Segments of the human population have developed resistance to tuberculosis over the thousands of generations of exposure, and so the evolutionary chess game will go on for the next century or millennium, as human hosts and their pathogens attack and counterattack.