After a few years, the epidemic waned, only to return in periodic waves for hundreds of years. Improved public health measures have reduced the threat of bubonic plague, but infected rats and fleas still thrive in some areas of the world. Environmental changes are factors in disease emergence. The increasing number of people infected with Lyme disease is partly due to the expansion of suburban communities into once-wooded areas inhabited by infected ticks. Hantaviruses became more apparent in the southwestern United States in part from weather changes that fostered increases in the rodent populations that carry them.
Microbes also reinvent themselves.
Influenza viruses are potentially deadly enemies that can mutate or change in ways that make them more virulent. Despite vaccines and other efforts to control them, these changing viruses still cause epidemics and worldwide pandemics. The worst flu pandemic on record occurred in 1918-1919 when millions died. Lesser outbreaks have caused an estimated half a million deaths in the last 20 years in the United States alone. This talent for reinvention also enables microbes to develop resistance to once-effective therapies, dramatically increasing human suffering and the costs of medical care.