For example, microbes travel and always have. In the 14 th century, the bubonic plague entered Europe as travelers following trade routes from Mongolia were accompanied by infected rodents and fleas. Eventually, rats brought the infection into North Africa and Europe on trading ships. 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.