MERS, H1N1, swine flu, chikungunya, Zika: Another virus with a peculiar name always seems to be right around the corner, threatening to become a pandemic.

Over the past decade, the World Health Organization has declared four global health emergencies. Two of them were in the past two years: the Ebola epidemic in West Africa and the Zika outbreak that's spread through the Americas.

If that seems like a lot, it is. Researchers who charted the rise of infectious diseases from 1980 to 2010 in the journal The Royal Society in 2014 found that outbreaks have indeed become more common in recent decades:

Many of the pathogens that spark deadly outbreaks aren't new. Researchers have known about Zika since the 1940s and Ebola since the 1970s. Some of these viruses have evolved with humans for hundreds or thousands of years.

But viruses, bacteria, and fungi can now spread around the world with greater effectiveness and speed than ever before. And when they turn up unexpectedly in new places, they catch doctors and health systems — and people's immune systems — off guard.

Here are four key reasons we're seeing an uptick in infectious diseases around the world:

1) More travel, trade, and connectivity

For most of history, humans lived in small, disparate bands that were relatively isolated from each other. "Only comparatively recently has there been extensive contact between peoples, flora and fauna from both old and new worlds," write researchers in a paper on global transport and infectious disease spread.

The rise of sailing in the 1300s helped spread deadly plague around the world through rat populations carried on boats.

And then the slave trade of the 16th and 17th centuries introduced Aedes aegypti — the mosquito type that today spreads viruses like Zika, yellow fever, and dengue — to the Americas from West Africa.

These pathogens spread at a relatively slow pace. It took more than 10 years for plague to spread across Europe, for example.

Air travel changed all that. "The jet plane took off in the '70s and accelerated during the '80s and '90s," said Duane Gubler, an infectious diseases specialist and former director of the division of vector-borne disease at the Centers for Disease Control and Prevention. "So now we have this modern transportation or globalization that is moving animals, humans, commodities, and pathogens around the world."

The movement of people and goods is happening at a faster rate and greater volume than at any other time.

You can now travel pretty much anywhere in the world in a day. And unlike the plague lurching across Europe in the 1300s, a traveler can now bring a deadly strain of bird flu from China to Europe within 24 hours.

When a pathogen is introduced to a new place, people are biologically more susceptible to the disease, since their immune systems have probably never been exposed and have no experience fending it off. Doctors and health systems can also be caught off guard.

This is one of the factors that helped the recent Ebola epidemic in West Africa spiral out of control: the three most affected countries had never experienced an outbreak of the virus before.

"Clinicians had never managed cases," the World Health Organization reported. "No laboratory had ever diagnosed a patient specimen. No government had ever witnessed the social and economic upheaval that can accompany an outbreak of this disease. Populations could not understand what hit them or why."

Contrast that with East Africa, which has had plenty of experience dealing with Ebola outbreaks over several decades. In Uganda, for example, as soon as an Ebola case is identified, public health officials overwhelm all streams of media with messages about how to stay safe. People won't leave their houses out of fear of infection, and they immediately report suspected cases to surveillance officials. It's one of the reasons Uganda has successfully stamped out about half a dozen Ebola outbreaks.

This new context helped spread an old virus around quickly, leading to more than 15,000 cases and 11,000 deaths.

2) Urbanization — "an emerging humanitarian disaster"

Not only are people and goods traveling farther and at a greater volume and speed than any other time in history, but people are also more likely to live in densely populated urban environments.

More than half of the world's population now lives in cities, and just about every country on the planet is becoming more urbanized. Global health researchers have called the trend "an emerging humanitarian disaster."

That's because most people don't live in relatively clean cities like Washington, DC, or Munich. "Most cities are unplanned, and many people — tens of millions — now live in crowded, unhygienic conditions," said Gubler.

Cities can be perfect breeding grounds for disease to spread. Consider the ongoing Zika outbreak in Brazil. Not only was this an old virus in a new country that caught health officials off guard but Brazil's many cities also happened to be extremely hospitable to the virus.

The Aedes aegypti mosquito, which carries Zika, thrives alongside people. As Gubler wrote in this 2011 paper, "[It's] a highly domesticated urban mosquito that prefers to live with humans in their homes, feed on humans and lay eggs in artificial containers made by humans." (Think tires or plastic cups.)

Across Latin America, 113 million people (nearly one in five) live in slums. Many of these slums lack a clean and steady water supply, so people keep buckets filled with water around their homes — ideal mosquito breeding grounds. Not to mention the fact that air conditioning isn't common, leaving bodies and homes warm and making them even more hospitable to the disease-carrying bugs.

Globally, unprecedented population growth following World War II has meant that not only are more people living in cities than ever before but populations are also exploding into areas that were once inhabited only by other animals.

Anytime humans interact with animals, there's a chance that a pathogen could make the leap across species and sicken them. Today about three-quarters of new emerging infectious diseases are spread to humans by animals — a health threat that came with the rise of agriculture.

As the historian Yuval Harari writes in his sweeping history of humankind, Sapiens: "Most of the infectious diseases that have plagued agricultural and industrial societies (such as small pox, measles, and tuberculosis) originated in domesticated animals and were transferred to humans only after the Agricultural Revolution."

Today, this is still the case, whether it's chicken sellers sitting on the streets of China risking exposure to bird flu or hunters in Guinea eating bushmeat that could be infected with the Ebola virus.

"That’s why many of these infections come out of Africa or Asia, where there's a strong link between humans, animals, and the environment," said Ali Khan, author of The Next Pandemic and the former director of the Office of Public Health Preparedness and Response at the CDC.

3) Pervasive poverty means outbreaks will be worse

When new viruses strike impoverished or weakened health systems, they have a much greater chance of thriving and killing people.

The 2014-'15 Ebola epidemic offers another illustrative example here. Every American infected with Ebola during that period survived. The same wasn't true for the affected West Africans, 11,000 of whom died.

The stark difference in outcomes had to do with money and access to health care: Patients with Ebola can be kept alive through tried-and-true health measures — kidney dialysis, IV rehydration, antibiotics — and 24-hour hospital care. While that's possible at the National Institutes of Health in Bethesda, Maryland, it wasn't in many of the places where Ebola struck, like Gueckedou, Guinea.

We're seeing a s