Pandemics, such as the 14th-century plague known as the Black Death, have been occurring for centuries. The Black Death devastated populations throughout Asia and Europe. And the influenza epidemic of 1918-19 caused at least 20 million deaths worldwide. AIDS, of course, can be found in almost every country.

Several factors contribute to the global spread of an infectious disease. First, it depends on how easily the disease-causing microbe is transmitted from person to person. For example, the tuberculosis microbe moves through a population much more slowly than the influenza microbe.

Some microbes live inside an animals, such as mosquitoes or mice, during part of their life cycle. The habitat and life cycle of that animal can limit or extend the range of the microbe.

Human behavior and public health conditions are also important factors. Reusing needles for injecting vaccines or drugs increases risk of infection, as does using water from a polluted source.