Will Disease Drive Us All to Extinction ?

Virulent infectious diseases and parasites have long been shown to be a significant cause of decline in biological populations. But can disease lead to the actual extinction of the host species—such as humankind?

Scientists attempt to determine the extinction-threatening effects of disease by first studying its role in historical extinctions. But proving that infectious disease is responsible for past extinctions is tricky business. After all, the extinct species is not around for scientific investigation. Even if a pathogen or parasite were discovered in a disappearing population, it would not prove that the pathogen itself was responsible for the decline.

However, reasonable evidence exists that historical extinctions and extirpations—local extinctions in which a speciesc eases to exist in the specific geographic area of study—are at least partlya ttributable to infectious disease. Avian malaria and bird pox are believed to have decimated certain bird populations in Hawaii in the late 19th century. In the mammal kingdom, the abrupt disappearance of native rats on Christmas Island in the Indian Ocean at the turn of the 20th century is believed to have beencaused by disease-carrying, flea-ridden black rats that arrived there on a merchant ship.

In recent years, numerous extant species have come under attack by invading infectious disease. In Australia, koalas are besieged by two major pathogens, one of which can cause sterility or blindness. The World Wildlife Fund claims that infections of these types could lead to the extinction of koalas within 50 years. Whether these or other species disappear remains to be seen, but research indicates that disease caused by pathogens and parasites is not likely to be the primary factor in the extinction-. threatening process. (Loss of habitat, human overhunting, and competition with new species are possible and/or contributing causes Emerging diseases such as Ebola, HIV/AIDS, SARS, and H1N1 influenza have wreaked worldwide societal havoc and resulted in tens of millions of deaths. Reemerging infectious diseases, which appear in new places or in drug-resistant strains, also pose a significant threat to human life. Among these diseases are dengue virus, West Nile virus, and even cholera, which affects 3 to 5 million people each year and causes more than 100,000 deaths annually—despite the existence of a safe and effective vaccine.

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