Tuberculosis posed a deadly threat to people in the ancient past. Its ability to spread among dense, malnourished populations made it a common problem for civilization. Since its collapse, the bacteria that causes the disease has had to adapt to stay alive, primarily by reducing its virulence so that its hosts live longer and can help spread it further. This has made it only rarely fatal, even though most people in the Fifth World carry the infection. In times of distress — for example, when food becomes harder to come by than usual — this can result in coughing and sneezing. The blood-containing mucus, fever, night sweats, and weight loss that characterized the disease in past centuries have disappeared.