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The infectious diseases that plague human societies emerged from natural ecosystems. Disease epidemics can often be linked to environmental problems and, sometimes, our misguided attempts to solve them.

On December 26, 2013, an 18-month-old boy in the village of Meliandou, Guinea, came down with a fever, began vomiting, and had bloody stool. Two days later, he was dead. Within a couple of weeks, several members of his family came down with a similar illness, which was later identified as Ebola. By then, the epidemic was in full swing, spreading to the neighboring countries of Sierra Leone and Liberia. As of July 2015, there were 27,678 suspected cases of Ebola and 11,678 deaths, according to the World Health Organization (WHO).

Ebola is a zoonotic disease, which is any infectious disease that can spread from animals to humans. According to the U.S. CDC, 6 out of 10 infectious diseases are zoonotic diseases. When humans come in close contact with wild animals through the pet trade or when they kill and butcher wild animals for so-called bushmeat, they are at risk of catching new and emerging zoonotic diseases.

Deforestation in tropical countries also puts humans in closer contact with wild animals. Scientists have found Ebola in chimpanzees, gorillas, antelopes, and porcupines; but, like many zoonotic viruses, they believe the natural reservoir is fruit bats. Although it’s not known exactly how the child in Meliandou caught Ebola in the first place, before he fell ill, he was seen playing near a hollow tree where bats roost. Another human-infecting virus harbored by fruit bats is severe acute respiratory syndrome, or SARS, which resulted in an outbreak in China in 2003 after it spread to a farm where wild civets were being raised for meat. Researchers also suspect that HIV was initially transferred to humans who came in contact with meat from chimpanzees or gorillas.

Diseases have evolved over millions of years to persist in species and ecosystems, and our efforts to eliminate them are often foiled by natural selection. One such disease is malaria, which is caused by a protozoan parasite carried by mosquitoes. Scientists have identified more than 100 species of malaria that infect birds, reptiles, and mammals. Six of these species are known to infect humans, primarily in tropical countries where malaria-carrying mosquitoes live. Each year, 300 to 500 million people become infected, and approximately 700,000 people die from the disease. Ninety percent of these cases occur in sub-Saharan Africa, where a child under 5 years of age dies from malaria every minute (Figure 11.15).

The best weapons we have against malaria after a person has already been infected are antimalarial drugs and pesticides that can reduce infection rates by reducing mosquito populations. However, both malaria parasites and mosquitoes are living organisms, and purely by chance some individuals within a population will be resistant to any new pesticide or antimalarial drug. When a pesticide or antimalarial drug is used, some of the resistant individuals will survive the treatment; those individuals will give rise to a new generation with greater resistance than their parents’ generation. This process repeats itself generation after generation, ultimately rendering the pesticide or antimalarial drug ineffective. Both these evolutionary processes have frustrated efforts to control malaria.

Quinine was an effective antimalarial drug during the 18th and 19th centuries, as European nations established colonies in tropical regions around the world. But by the mid-20th century, resistant strains of malaria in many areas forced quinine to be replaced by chloroquine. Resistance to chloroquine has now become common in sub-Saharan Africa, Asia, and South America, and attention has turned to a compound called artemisinin, which is extracted from a plant called sweet sage. The first appearance of reduced susceptibility of the malaria parasite to artemisinin was reported in 2011. The team reviewing the international campaign to eradicate malaria wrote: “Losing . . . [artemisinin] to resistance would be a disaster for the control of malaria and would bring eradication efforts to a standstill.”

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Reports in a 2012 issue of the medical journal The Lancet further emphasized the point: “Antimalarial control efforts are vitally dependent on artemisinin combination treatments. Should these regimens fail, no other drugs are ready for deployment, and drug development efforts are not expected to yield new antimalarials until the end of this decade.”

If the parasite can’t be eliminated, is it possible to eliminate the mosquitoes that transmit the parasite? In the 1950s and 1960s, the WHO conducted an anti-malarial campaign that employed widespread application of DDT, often from trucks that cruised through neighborhoods every evening spraying DDT. Not surprisingly, mosquitoes responded by evolving resistance to DDT, and within a few years the effectiveness of DDT had declined in many parts of the world. Other insecticides replaced DDT, but, in a repetition of the earlier events, targeted mosquito populations are rapidly evolving resistance to the new chemicals.