The plague is not new to Madagascar. The disease re-emerged in the country in the 1990s. And now Madagascar reports more cases than any other country — about 300 to 600 each year.
People catch the plague bacteria — Yersinia pestis— from fleas that live on rodents. So the disease thrives in cities with poor sanitation.
After a coup d'etat in 2009, Antananarivo's health and sanitation systems collapsed, Aaron Ross wrote on the Pulitzer Center's website in January. "Trash can go weeks, even months, without being collected and rats have become a common sight along the narrow alleyways that coil around the city's steep hillsides," Ross wrote.
The plague's signature symptom is large, swollen lymph nodes. This form of the disease is called bubonic plague. And it's not very contagious.
When the infection moves to the lungs, it's called pneumonic plague, a form that's more dangerous. It kills quickly, and it spreads from person to person through coughs.
In the current outbreak, so far, only 2 percent of the cases are pneumonic, WHO says.
Both forms of the plague are easily cured with antibiotics when the disease is caught early.