The proliferation of drug-resistant strains of many pathogenic organisms is an increasing public health problem. Many pathogenic organisms develop antibodies, enzymes, or other metabolic means ofadaptation to drugs such as antibiotics that initially are efficacious. For example, staphylococci, gonococci, and pneumococci have all evolved an enzyme that denatures penicillin, rendering it useless. The bacillus of tuberculosis has evolved strains resistant to all the first-generation drugs developed to combat it. The malaria parasite has evolved strains that resist antimalarial drugs. Insecticides such as DDT that were initially 100 percent effective in destroying insect vectors such as the anopheline mosquito are similarly rendered inefficacious. This has happened because of widespread, often indiscriminate use of these drugs (and insecticides). Not all the exposed pathogens are killed, and those that survive selectively breed to produce resistant strains.