"As people go further and further into these rain-forest-type areas, they seem to come more in contact with the reservoirs of the virus," said Robert Garry, a professor of microbiology and immunology at Tulane University who has worked extensively in West Africa. "In this case, it looks like [people in Guinea] just cut far enough into the forest to find a reservoir."

A variety of animals carry viruses that can be transmitted to humans, a process known as zoonosis. While bats are the most likely culprits in this outbreak, gorillas and other mammals can also carry Ebola. As population centers grow and infrastructure expands, researchers like Epstein and Garry say this type of human-animal contact will be on the rise.

One study, conducted by African and U.S. researchers in 2012, found that some kind of human activity often immediately preceded Ebola outbreaks. A 1994 outbreak, for example, took place after gold miners in Gabon ate an infected gorilla.

Deforestation is rampant in West Africa. In Sierra Leone, all but 4 percent of the country’s original forest cover was wiped out by the 1920s, according to the United Nations Environment Program. The small amount of forest left is in danger of disappearing by 2018, according to the U.N. In Liberia, agriculture has eaten up about 20 percent of the country’s dense forests since 1979, according to a government report. Guinea has lost about 20 percent of its forests since 1990, according to U.N. data.

Those changes to the environment can increase concentrations of wild animal populations in the remaining forest, bringing previously untouched reservoirs of Ebola into closer proximity with humans.

And development has increased the mobility of people in Africa. As countries like China have poured billions into developing West African mineral deposits, new roads and infrastructure have cropped up in once remote villages. That brings new opportunities not just for people to travel to previously hard-to-reach areas but also for increasing contact with those who may be infected with Ebola.

“It used to take 12 hours to drive from Freetown to Kenema,” the third-largest city in Sierra Leone, Garry said. “Now it takes four, and people are used to moving around. That makes it harder to contain the virus.”

There is also evidence that climate change could make Ebola outbreaks more likely by causing periods of extreme drought and extreme rain. Outbreaks have been correlated with intense periods of heavy rain that come after periods of aridity.

“Extremely dry conditions force some fruit trees to defer fruiting. When the rains come and the stricken trees put out fruit, all manner of fruit-starved species, including Pteropus bats and apes, gather to feast,” explained science writer Sonia Shah in a Yale University article. “Large numbers of creatures concentrated under newly fruit-heavy trees provide microbes such as Ebola a prime opportunity to jump from one species to another. And once Ebola starts circulating heavily in a new species such as apes or bats, it can readily be transmitted through infected blood and other fluids to humans.”

And climate change could force people to resort to different, possibly more disease-ridden sources of food.

“We already know climate change is weakening crop yields,” said Kristie Ebi, a professor of global health at the University of Washington. “When there’s high food insecurity, how will people go about making sure that they have enough food for their families?” If people in poor, rural sections of West Africa have less food to eat, they may be forced to eat bats and other animals that can carry Ebola, Ebi said.

Experts say it’s critical for West Africa’s public health infrastructure to be developed in order to prevent outbreaks that could become more frequent.

In many parts of West Africa, health education, hospitals and disease-tracking systems are sorely lacking. Scientists say better development in those areas could have prevented the Ebola outbreak that began in Guinea.

“If we were able to do surveillance and successfully quarantine, then we wouldn’t have this problem,” said Aaron Bernstein, associate director of the Center for Health and the Global Environment at Harvard University. “We know how to control it. We know what animals carry it. We know how to screen for it. We know how to quarantine. So the question is, Can we do all that? Do we have the international will?”