Public-health workers had nearly eradicated polio in the 20th century. So why is it making a comeback now?

Polio causes paralysis in about 1 in 200 cases

This week, the World Health Organization declared a Public Health Emergency of International Concern — the second ever since the classification was created in 2005. The reason? Polio is spreading between countries — and during a time of year when cases are usually low. Already in 2014, there have been 74 confirmed cases of wild poliovirus, which causes paralysis in about 1 in 200 infections.

The news underscores a grim reality about polio. The World Health Organization has the money and the vaccines it needs to stamp out the disease out once and for all — but it hasn't been able to finish the job. A major obstacle is Pakistan, where heavy opposition to vaccination programs in some regions has let polio regain a foothold.

And there's a catch — the longer Pakistan takes to eradicate the virus, the more likely it is that other countries will let their guard down. If that happens, a major outbreak is entirely conceivable.

We thought we had polio under control

The fight against polio has been one of the big public health success stories of humankind — with the number of reported wild cases dropping from 350,000 in 1988 to 406 confirmed cases in 2013. This was achieved through a multi-billion-dollar international effort that has vaccinated billions of children.

Indeed, the World Health Organization was so encouraged by the progress that they'd pledged to eliminate polio altogether by 2018. But those plans have now hit a snag.

Now polio is making a comeback

Over the past year, progress seems to have stalled. By the end of April in 2013, there were 24 confirmed cases of wild polio. By the same time in 2014, there were already 68:

68 cases may not seem like a big number — but it's a big deal. That's because very few people with the infection actually show symptoms (and because not every case of paralysis from polio gets officially diagnosed and confirmed). So behind every confirmed case are hundreds of other people carrying the virus and spreading it.

A big outbreak during the winter is bad news when the summer hits

Although some variation from year to year is normal, experts are particularly concerned because of transmission between countries. This year, it's happening during the cold months, when polio tends to spread slowly. The disease then propagates more rapidly in warmer months when there's more rain and contamination of water through sewage.

"If we have this much transmission in low season, what's going to happen in high season is it's going to multiply exponentially," says Christy Feig, the director of communications at the World Health Organization.

The WHO's official recommendations include immunization checks for people leaving Syria, Cameroon, and Pakistan, which have spread a few cases to other countries. Nigeria, which has historically had a major problem, seems to be doing much better, with two cases through the end of April.

The big problem is in Pakistan — where vaccination efforts have stalled

There's one country that seems to be at the epicenter of the latest polio revival: Pakistan.

59 out of 74 known cases of polio are in Pakistan

59 out of 74 known cases of polio are in Pakistan, according to the most recent numbers, which the WHO revealed in a press conference on May 5. (The next highest count is Afghanistan, with four. Those cases came across the border from Pakistan.)

Not only is polio increasing inside Pakistan's own borders, the country has also been exporting the disease. Polio strains that originated in Pakistan have spread cases to Afghanistan, Syria, and Iraq. (The latter two states had been polio-free for more than a decade.) They've also turned up in the sewers of Israel and Egypt, but without any confirmed cases of people getting ill.

So what's going on? One big cause is that vaccination in Pakistan has stalled.

"It’s primarily a security problem," says Steve Wassilak, a medical epidemiologist at the Global Immunization Division of the US Centers for Disease Control and Prevention. "There’s a bit of disenfranchisement of the Pashtun community, but it’s relatively minor when compared to the issue of actual security in the area."

Security matters seriously complicate vaccination efforts in the Karachi metropolitan area, Khyber Pakhtunkhwa (formerly known as the North-West Frontier Province), and the semi-autonomous Federally Administered Tribal Areas (where many 2014 polio cases are occurring). The Taliban has a strong presence in each area.

Vaccine workers have been targets of violence — dozens have been killed in targeted attacks.

And vaccinators can't even try to operate in some places altogether. In 2012, a Taliban commander banned vaccinations against polio in North Waziristan until drone strikes stop. Vaccination has also been essentially forbidden in South Waziristan and parts of Khyber Pakhtunkhwa. (Both North and South Waziristan are in FATA.)

This general area tends to be more wary of polio vaccination programs, including rumors that the vaccine violates Islamic law or is a Western plot that causes sterilization. The revelation in 2011 that the CIA used a fake Hepatitis B vaccination program as part of its hunt for Osama bin Laden certainly wasn't of any benefit to the polio program, either. "It isn't the cause of where we're at with polio right now, but it certainly hasn't helped and has made the situation in Pakistan weaker," says Heidi Larson, an anthropologist who studies public trust and vaccines at the London School of Hygiene & Tropical Medicine and is the former Global Communication for Immunization at UNICEF .

Whatever the causes, public-health experts say the situation in Pakistan has reached a critical stage. "If Pakistan doesn’t step up the game and do their part, we are going to lose control of the effort to control polio," WHO director of communications Christy Feig told me. "In some areas they have stopped vaccination campaigns, and they need to start increasing those vaccination campaigns in order to get the virus under control."

How polio could make a comeback worldwide

A few dozen cases in Pakistan may not seem like the biggest deal. But, in many ways, this is a race against time. The longer it takes for Pakistan to eradicate polio, the more likely it is that the disease could spread around the world.

In many ways, this is a race against time

Here's why. Many countries have already eradicated polio. But, over time, people in those countries are likely to get lackadaisical and stop vaccinating, thinking that polio is no longer a threat. Those unvaccinated populations — including millions of children in Europe — would then be extremely susceptible to the virus if it ever entered their borders.

The World Health Organization says that it's still on target to finish off polio for good in 2018. But that depends largely on vaccination programs in Pakistan. And if those continue to stall, the risks of a worldwide outbreak could grow.

Update: Added a statement from the CDC and additional details about security issues and vaccination bans in Pakistan.