The number of people who live in Asia is greater than the total number of people who live in the rest of the world. More than 160 cities in Asia have a population of at least one million people. Thus, when new infectious diseases threaten popu- tions in Asia, huge segments of the global population are at risk. At the same time, Asians are thoroughly integrated with the rest of the world, providing skilled exp- tise and becoming trading partners in all continents. Infectious diseases ordinarily show no preference for infection or disease according to race or ethnic background. A few exceptions exist, due to the host- pathogen evolution that happened before the recent era of rapid travel. Such exc- tions occur usually because the infectious agent was newly introduced to one population only after having existed and evolved for hundreds or thousands of years in a different population. As air travel became popular in the last few generations of people, it became increasingly difficult for populations to remain in isolation. Thus, in 2003, SARS in China rapidly became SARS in Canada. Throughout history, a major source of new infections of people has been old infections of animals. For some, such as Ebola or Lassa, transmission to people is rare and self-limiting, though frighteningly lethal for the few unfortunate indivi- als who get infected. And Ebola and Lassa are indigenous for Africa, not Asia.
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