Plague is spreading at an alarming rate in Madagascar. Yes, plague.  - Vox

People in Madagascar are scrambling to get their hands on antibiotics and face masks, while public gatherings have been canceled, as a rare pneumonic plague epidemic spreads quickly across the country.

Plague is the deadly flea-borne bacterium you might associate with the Middle Ages, when the “Black Death” killed 50 million people, wiping out most of Europe’s population. But it's still around today, even in the United States.

In some countries, particularly Madagascar, plague is endemic, and flare-ups sometimes cause public health emergencies — like the one that’s quietly brewing there right now.

As of October 6, 231 plague cases there had been identified, as well as 33 deaths, according to the World Health Organization. Twenty of Madagascar’s 114 districts are now affected by the epidemic.

Madagascar typically sees about 400 cases each year. But what makes this outbreak worrying is that plague has now spread through the capital and to coastal cities, “which we have not seen before,” World Health Organization spokesperson Tarik Jašarević told Vox.

Also unlike previous outbreaks, this year’s involves mostly pneumonic plague, a more dangerous form of the disease than the much more common bubonic plague.

Pneumonic plague attacks the lungs and spreads from person to person through droplets from coughing, like a cold. (Bubonic plague spreads only from fleas to humans.)

“So it’s a worrying situation because it’s in an urban setting, and it’s transmissible from human to human,” Jašarević added. “We need to act quickly — to trace the contacts of those infected and put people on a preventive treatment.”

Some 30 to 60 percent of people with bubonic plague die, while untreated pneumonic plague is always deadly, typically within 24 hours of disease onset. A timely antibiotics dose can, however, save lives, and that’s why the WHO delivered an emergency stockpile of nearly 1.2 million doses of antibiotics to the Indian Ocean island nation.

There’s a low risk that plague will spread internationally

Doctors and nurses from the Ministry of Health and officers of the Malagasy Red Cross staff a “health care checkpoint,” trying to inform passengers leaving Antananarivo about plague and to potentially detect suspected cases. RIJASOLO/AFP/Getty Images

Health officials traced Madagascar’s current plague outbreak back to a 31-year-old man from Toamasina, a coastal city in Madagascar. On August 27, the man used public transport to travel about 500 miles, through Madagascar’s capital, Antananarivo, and died while en route.

Public health workers found out that many people he was in contact with had come down with plague, and they had spread the disease to others too. Since then, cases have been popping up elsewhere around the country.

Plague is caused by the bacterium Yersinia pestis. Wild rodents — chipmunks, mice, squirrels — can carry the bacteria. Fleas feed on the rodents and pick up plague, in turn spreading it to other mammals, including humans, through bites.

When people talk about plague, they're usually talking about bubonic plague, the type that causes about 80 percent of infections in the US every year. Bubonic plague infects the lymphatic system, causing the lymph nodes to swell up and feel tender. These swellings, known as buboes, typically pop up in the groin, neck, and armpits. A blackish pustule will also arise at the site of the flea bite, and hemorrhaging can occur underneath the skin, causing it to blacken — part of the reason the plague is called "Black Death."

When bubonic plague is left untreated, it can spread into the lungs to become pneumonic plague, or into the bloodstream to become septicemic plague. These types of plague are more rare — and more dangeous — because unlike bubonic plague, they can spread from person to person.

And again, it’s pneumonic plague that’s affecting Madagascar right now.

The WHO believes there’s a very low risk that plague will spread internationally from Madagascar — though the risk that it’ll continue to brew within the country is high.

It’s also really, really early in the epidemic season to see such a high burden of illness, since the disease typically surfaces from September all the way to April.

And that’s a situation worth keeping an eye on, as infectious diseases expert Crawford Kilian warned on his blog. “Pneumonic plague in a big city is like Ebola in a big city: better to prevent it than suppress it, and prevention now seems out of the question.”

In much of the world, plague is associated with poverty

Houses in Antananrivo, Madagascar, on October 2, as plague spreads rapidly in cities across the country. Henitsoa Rafalia/Anadolu Agency/Getty Images

As this study in PLOS Medicine reports, poorer countries with bad sanitation and other risky practices can help spread the infection:

Africa is particularly at risk for a number of reasons. Poor rural communities typically live in close proximity to rodents, which are widely hunted and eaten in many plague-endemic areas. Superstitions, lack of money, and distance from health facilities often lead to delays in seeking health care and receiving treatment. The public health system in large parts of Africa is poorly organised and equipped, and political crisis and social disorganisation impede improvements.

So it’s not surprising that on the global scale, plague is most common in poorer countries, such as Madagascar, where the per capita GDP is only $1,500. Plague turned up on the island in 1898 through trade routes, and outbreaks have popped up nearly every year since 1980. (For the past two decades, Madagascar has experienced the highest burden of plague cases in the world.)

But there are other factors that help spread plague: Humans have been encroaching on wildlife areas, putting them into contact with potential carriers of the disease. And this is as true for poorer countries as it is for developed places like the US. As this scientific study on US plague reports, "Plague in New Mexico has increasingly occurred in more affluent areas" — the result of building out suburban and exurban communities in previously underdeveloped areas where plague had been circulating in wild animals.