People not only get around faster than ever before, but they change the environment more easily too.
Every day, in some part of the world, whole tracts of rain forests are bulldozed, rivers are dammed, and new roads are paved into the wilderness. This disrupts the balance and distribution of plants and animals, including microbes. It may cut off a virus from its host so that the virus must seek another means of survival.
The story of Lyme disease, which causes arthritis-like aches and pains, provides an example of this process. In the 1800s settlers in Old Lyme, Connecticut, clear-cut the old growth forests, which led to a decline in the deer population. A hundred years later, when the agricultural production in that area ceased, the forests returned, along with a burgeoning deer population. But the human population grew too. Housing developments in forested areas put man, deer, and microbes on a collision course.
The spirochete Borrelia burgdorferi is passed from a deer to a deer mouse by the bite of an infected deer tick. The deer and the deer mouse do not seem particularly affected by the microbe, but people are. When people started to build houses in Old Lyme, they unwittingly placed themselves in the path of the microbe and added a new host to the microbe's list.
Even making more subtle changes to the landscape— such as digging pools, opening irrigation ditches, and discarding tires—create new niches for vectors, animals that are capable of carrying human disease. Insects carry about one hundred different human diseases, which are called arboviruses (arthropod-borne viruses). Topping the list are yellow fever, dengue fever, malaria, and West Nile virus.
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