Logging, hunting, farming, and mining in forested areas alter their delicate biological equilibrium. At times that is good for human health, but it can also have the opposite effect. Among other things, "Biodiversity: Its Importance to Human Health", edited by Eric Chivian of the Harvard Medical School, explains how that can happen.
Human activities change these areas’ temperature, how humid they are, their predator populations, and their vegetation. That sometimes increases the populations of mosquitoes, flies, mice, bats, and other vectors of infectious diseases. Settling near the forests’ edge can expose people to diseases found there such as malaria, yellow fever, leishmaniasis, Chagas disease, and African sleeping sickness. Increased consumption of wild meat can facilitate the spread of diseases from animals to people.
Logging has made malaria more common in some Southeast Asian and Amazonian regions. It created new pools of standing water and made others less acidic, which favored the mosquitoes that spread malaria.
The smoke from Southeast Asia’s massive forest fires in 1997/98 probably made many trees fail to flower and produce fruits. To find food, many fruit bats moved to the fruit trees in Malaysia’s large pig farms. These bats passed on the deadly Nipah virus to the pigs, which then transmitted it to humans. That forced the government to destroy huge numbers of pigs.
Forest destruction may have provoked the Lyme disease epidemic in the northeast United States. It caused many animals that eat white-footed mice or compete with them to disappear and the mouse population to grow. These mice spread the bacteria that cause Lyme diseases to ticks, which passed them on to people.
Three-quarters of new emerging human diseases come from animals. People contracted many of these diseases by eating the meat of wild animals that carried them. This may have been how HIV/AIDS appeared and there is a high risk that similar viruses may be passed from primates to people in the future. Eating wild meat has also been linked to outbreaks of anthrax and the plague.
Forest fires and deforestation contribute to global warming. That has been linked with the spread of dengue fever, malaria, yellow fever, and encephalitides to regions where these diseases previously did not exist.
Disturbing forest ecosystems does not always affect health negatively as in these cases. However, it happens frequently enough that it deserves our attention, because the way people are destroying forests is enough to make you sick.
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The full reference for the document is: Chivian, E. (editor). 2002. Biodiversity: Its Importance to Human Health, Interim Executive Summary, Cambridge, MA: Center for Health and Global Environment, Harvard Medical School.