Last week, the bubonic plague — known for wiping out nearly one-third of Europe’s population in the Middle Ages — infected some 20 people in a village in Madagascar, an island situated in the Indian Ocean off the southeastern coast of Africa. The non-profit group the Pasteur Institute of Madagascar confirmed that all 20 of those had died, according to the BBC.

The International Committee of the Red Cross (ICRC) warned of the possibility of an outbreak on the island of Madagascar back in October. After a coup in 2009, conditions and living standards worsened in villages and towns on the island, due to political crisis, according to the Pasteur Institute. Health experts were particularly worried about sanitation in prisons. “If the plague gets into prisons, there could be a sort of atomic explosion of plague within the town,” the Pasteur Institute’s Christophe Rogier told the BBC. “The prison walls will never prevent the plague from getting out and invading the rest of the town.” This is due to the fact that a giant rat population lives in the main prison in the capital city of Madagascar, among 3,000 inmates. The plague is carried through rats and infected fleas.

The bubonic plague, known as the Black Death in the Middle Ages, killed some 25 million people in Europe in the 1300s — about one-third of its population. The disease is spread to humans through rat or flea bites, though it’s not contracted from human to human; it is caused by the bacterium Yersinia pestis. The World Health Organization simply refers to the disease as “Plague,” though it notes that humans can develop the bubonic form when their lymph nodes swell, leading to “buboes.”

The outbreak on Madagascar is one of the worst in recent years, though last year a total of 60 people died from the plague on the island. The bacterium was introduced to the island in 1898, and since then outbreaks occurred every so often – except for a hiatus starting in the 1920s and lasting until another outbreak in the early 1990s.

Earlier this year in August, a 15-year-old died of bubonic plague in Kyrgyzstan, becoming the first case in Kyrgyzstan in 30 years. In the past two decades, other countries have also experienced outbreaks, including India, Indonesia,, and Algeria. Though the plague was deadly in the 1300s, when caught early today it can be treated effectively with antibiotics. Health officials will be investigating the prison area and other cities in Madagascar to prevent further cases.