Cases of monkeypox are on the rise in the U.S., with about 67,600 global cases, including about 25,500 in the U.S. Simultaneously, the world is still facing a COVID-19 pandemic, despite the number of cases tapering off.

Researchers say these types of viruses, known as zoonotic diseases, or ones that spread between humans and animals, will become increasingly commonplace as factors such as the destruction of animal habitats and human expansion into previously uninhabited areas intensify.

Humans and animals are interacting more

Monkeypox was first found in monkeys in 1958 and in humans in 1970, according to the Centers for Disease Control and Prevention.

Elements such as deforestation, population growth and animal breeding have removed the boundaries between where humans and wild animals live, bringing them into closer contact.

Since 1990, about 1 billion acres of forest have been cut own to make room for other uses. Deforestation rates have been decreasing, with an average of 25 million acres being cleared each year from 2015 to 2020, down from about 40 million per year in the 1990s, according to a United Nations report.

Besides the impact on the climate, deforestation means a loss of habitat that often ends up driving wildlife nearer to people.

"You're just seeing the effects of the change in the environment, the change in animal behavior, the change in human behavior, bringing wild animals and humans more into contact where they can have more contamination," said Lanre Williams-Ayedun, the senior vice president of international programs at World Relief, a sustainability nonprofit organization.

Those changing patterns in animal migration and reproduction can influence how pathogens behave in their natural host, possibly becoming more contagious in the process, said Dr. Carl Fichtenbaum, the vice chairperson for clinical research for internal medicine at the University of Cincinnati.

"Depending on the particular germ, when it has an opportunity to do this multiple times, the germ adapts to the new species," he said.

A United Nations study found an estimated 60% of known infectious diseases found in humans and 75% of all emerging infectious diseases are zoonotic, or transmitted between species, from animals to humans.

Some of those include Ebola, Zika and COVID-19, which scientists hypothesize started in bats.

Could the current monkeypox outbreak have been predicted?

Monkeypox is endemic, or regularly found, in some African countries. But because monkeypox can be "self-limiting" and not as transmissible as other viruses. "It wasn't something that you would have thought would become such a big outbreak," Williams-Ayedun said.

The virus was nearly eradicated at one point when people in those regions received vaccines for smallpox, a relative of monkeypox, in larger numbers. But now, vaccine rates are much lower in people 40 and younger, Williams-Ayedun said.

People are also traveling farther and more frequently these days.

"It's easy to spread diseases globally, and we've seen that something that happens in what we think is a remote part of the world somewhere can very easily become something that is a concern where we live," she said.

Luis Escobar, an assistant professor in Virginia Tech's fish and wildlife department, said that while researchers have been able to predict where small outbreaks of monkeypox are more likely to occur – poorer regions, areas with war or social conflict or remote places — it is in those places where data is less accessible.

"My perception is that the data may not be enough," he said. "The data may have not been enough to anticipate a global epidemic of this magnitude."

He added that scientists must survey zoonotic diseases "in all corners of the world because we don't know which [region] is going to trigger the next pandemic."

Fichtenbaum agrees, and said that with the thousands of germs in the ecosphere, it's hard to know which ones will spread to pandemic-level proportions.

"I think it would be really disingenuous if someone says, 'Well, I can predict that this germ is going to be the next big germ,'" he said. "I think we're not very good at that, in the same way that we're not very good at predicting earthquakes."

The spread of zoonotic diseases will likely become more frequent

Escobar said that in looking to the future, researchers have neglected past data in their work to combat disease spread.

"The research I do is a bit to anticipate the future," he said. "But we're putting a lot of effort to try to reconstruct the past. We're analyzing data from the last century – in terms of wildlife diseases, climate, forest laws in the last 100 years – and with that, we are understanding what is happening now."

He and his colleagues have used that data in simulations to predict patterns in the next 50 to 100 years. But zoonotic diseases may not need that long.

Escobar's research suggests in the next 12 to 20 years, there could be a significant increase in diseases spread to humans from bats. Diseases endemic to Latin America's bat population could begin making their way to the American South as Latin America gets warmer, he said, which affects the distribution of and quantity of bats.

Additionally, diseases that are only exclusive to animals could tell us a lot about what society might look like down the line.

For example, as global warming continues to intensify, a virus common among fish could decimate aquaculture, causing blows to food production and the economy, Escobar said.

What can be done about it?

Fichtenbaum says public policy will need to address the spread of zoonotic diseases.

"I think right now, much of the climate change focus has been focused on, 'Well, this is bad for the environment, and we're going to see floods, and we're going to see heat waves, and this may affect economic survival.' But people aren't always looking at it in terms of health and human disease, which is very costly."

In recent years, some researchers in the zoonoses field of study have been pushing toward a "one health" approach, the merging of public health, veterinary health and environmental health, Ayedun-Wliliams said.

Helping people secure jobs, safe shelter and food is also important, as scarcity can result in hunting wild animals or cutting down trees for homes, and in turn, drive zoonotic diseases, she said.