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 similar story play out right now with an outbreak of yellow fever in Angola. More than 2,500 people have been infected with the virus, and 300 have died, in an ongoing outbreak.

This outbreak could have been prevented. While there's no cure for yellow fever, a vaccine was developed in 1936, and it's highly effective. Within three or four weeks after receiving the shot, nearly all people are protected from the virus. But in order for the vaccine to really prevent outbreaks, many people need to be immunized.

That's not happening in many parts of Africa, including Angola, where the virus is endemic. Starting in 2006, the World Health Organization, with support from Gavi (an international organization focused on improving vaccine access), ramped up efforts to make sure at-risk communities got vaccinated. But many countries on the continent still have vaccine coverage rates that are much too low to make the vaccine effective.

So because of poverty and weak health systems, even when we have the technology to stop disease spread, we don't get to use it.

"Politics and social factors play a determining role in whether or not you have one or two cases — and whether or not you have an outbreak or pandemic," said Khan. "We probably can't prevent the one to two cases. But we sure as heck can prevent the pandemic."

4) A warming climate is helping fuel more disease outbreaks

When we think about health, experts say, we need to start thinking about how environmental factors like climate change can matter as much as — or sometimes even more than — our personal behaviors.

In a report released in June 2015, The Lancet brought together the world’s leading experts on environmental health. They argue that "[t]he implications of climate change for a global population of 9 billion people threatens to undermine the last half century of gains in development and global health," including the spread of disease vectors.

For example, Zika, dengue, and chikungunya are all spread by the Aedes mosquito. And one of the reasons researchers think Aedes may be reaching new places —and more people — lately is climate change. (Mosquitoes thrive in warm and moist environments.)

Bird flu, cholera, Lyme disease — researchers believe all are being made worse by climate change.

In his decades as a disease detective for the CDC, Khan has witnessed the expansion of vector-borne diseases in the US. "It’s already happening now," he said. "And it's only going to continue to accelerate as our climate continues to get warmer, as we continue to have these extremes in rain fall, and weather events."

Despite all this, we're getting better at stopping outbreaks

The researchers who published on the rise of infectious disease outbreaks in The Royal Society also found that while the number of outbreaks was increasing globally, the number of outbreak cases per capita was actually declining over time: "Our data suggest that, despite an increase in overall outbreaks, global improvements in prevention, early detection, control and treatment are becoming more effective at reducing the number of people infected."

The researchers I spoke to also mentioned that we've generally gotten better at detecting outbreaks and advancing medical technologies — vaccines, medicines, diagnostics — needed to control spread.

Where we fail, they all said, was in strengthening public health systems globally to reduce the risk of a couple of cases turning into something much bigger and deadlier.

For example, the vector control programs that started after the WWII, Gubler pointed out, have been victims of their own success.

"Health authorities couldn't see any sense in continuing to spend a lot of money to control diseases that weren't occurring, so the programs were disbanded," he explained. "At the same time, many countries disbanded their public health infrastructure to deal with vector-borne diseases." This is another reason mosquito-borne diseases like yellow fever, dengue, and Zika are on the rise.

The money spent on public health has been in a steady free fall in the US in recent years. When health emergencies like Zika or Ebola hit, there's no emergency funding mechanism to quickly get a response in place. And at the global level, the budget of the World Health Organization has been at a standstill and not adjusted for inflation for years.

Yet what stopped the SARS and Ebola outbreaks from truly going global were simple, old-fashioned public health measures like contact tracing and quarantines.

"We continue to invest in the latest technology or gadget in order to prolong our lives by a few more days — stem cell therapies, personalized medicine — when a lot of our health depends on public health," said global health professor (and Vox contributor) Steven Hoffman.

Investing in public health means having strong disease surveillance systems and lab networks in place, public health officials ready to coordinate emergency health responses, and research capacity to quickly develop outbreak countermeasures like medicines and vaccines.

We're long overdue for a catastrophic flu pandemic, Hoffman added. "Our luck will run out. But in the meantime we need to ensure limited resources are allocated effectively."