A brief history of travel and the movement of microbes

Throughout history, travelers have carried microbes to new geographic areas and susceptible populations and provided the spark that could ignite epidemics. Recurrent bubonic plague occurred along the Silk Road as early as the 600s. Plague followed the routes of trade caravans in medieval Europe over the years 1347-52, killing an estimated 20 million people (Cliff et al., 2004). It reached Europe from Central Asia, arriving at Kaffa on the Black Sea in about 1347; from there it was carried by ships to the major ports of Europe and North Africa, and then spread over land routes. Explorers introduced smallpox, measles, and other infectious diseases into the New World, causing an estimated 56 million deaths (Black, 1992) and contributing to the collapse of the Aztec and Inca civilizations (Crosby, 1972).

Although Columbus is described as discovering the New World, the historian Carmichael describes the event as the creation of one new world from two old worlds. Cultural as well as biological transformation of the world followed the expanded movement of European overseas trade and exploration (Carmichael, 2006). Global human disease patterns also changed after 1500.

Aided by the modern steamship, which carried infected rats and fleas, a third bubonic plague pandemic circled the globe between 1884 and the early 1900s (Echenberg, 2002). Carried in grain wagons and other forms of transport, it moved inland from the port cities across southern China, reaching Hong Kong in 1894, Singapore and Bombay in 1896, Alexandria, Oporto, and Honolulu in 1899, Sydney, Buenos Aires, Rio de Janeiro, and San Francisco in 1900, and Cape Town in 1901. Many port cities had overcrowded urban tenements with impoverished populations, often immigrants, who were most often infected and were subjected to isolation, rejection, and stigmatization (Echenberg, 2002).

In 1787, the time to travel from England to Australia by sailing vessel was about a year (Cliff and Haggett, 2004). Many infections transmitted from person to person, like measles, were no longer a threat by the time of arrival because passengers were dead or immune. After 1860 the faster steamship quickly replaced the sailing vessel, and with that technological advance came shorter boat trips that increased the risk of transmitting infectious agents across oceans. The time to travel from England to Australia had dropped from 100 days (by clipper in 1840) to 50 days by the early 1900s. The development of relatively inexpensive air travel further increased the risk of spreading infectious agents across vast distances. Now a traveler can reach almost any major city on Earth within 24 hours - less time than the incubation period of most infectious diseases.

Last century, the Spanish flu of 1918-19 spread around the world in three waves, carried by humans, and killed as many as 50 million people (Barry, 2004). In the latter decades of the twentieth century, the virus that causes AIDS was carried throughout the world with human travelers as the primary transporters, who disseminated it to all countries. The best evidence suggests that the virus emerged after multiple introductions of related simian viruses in African monkeys and apes into the human population and the subsequent evolution to the human immunodeficiency virus (HIV) (Hahn etal., 2000). The connectedness and ease of travel in the late twentieth century allowed it to spread widely before the magnitude of the threat was recognized. Infections that can be transmitted from person to person can be carried by travelers to any part of the Earth (Wilson, 2003b).

The number of plant and animal species is higher in tropical areas and decreases as the distance from the equator increases; this is known as the latitudinal species diversity gradient. A recent analysis of species that cause infectious diseases (including parasitic species) shows a link between latitude and the spatial pattern of human pathogens, and suggests that climatic factors play a primary role in this pattern (Guernier et al., 2004).

Even today, the spectrum of disease from infections varies in relation to place. A recent analysis of sentinel surveillance data on >17,000 ill, returned travelers seen at GeoSentinel sites (staffed by clinicians knowledgeable about clinical tropical medicine) found significant regional differences in proportionate morbidity for 16 broad syndromic categories (Freedman et al., 2006). Systemic febrile illness occurred disproportionately among travelers returning from sub-Saharan Africa and Southeast Asia.

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