(CNN) An outbreak of the plague has killed dozens in Madagascar, and experts fear those numbers could go up.

At least 119 cases were confirmed by late last year, including 40 deaths, the World Health Organization said in a statement.

And the disease is taking an alarming turn.

"The outbreak that started last November has some disturbing dimensions," the WHO said this week. " The fleas that transmit this ancient disease from rats to humans have developed resistance to the first-line insecticide."

It's especially spreading in densely populated slums in the capital of Antananarivo.

Cases were confirmed in at least 20 districts and the capital, Christophe Rogier of the Institut Pasteur de Madagascar said late last year. Rogier is part of a team working with the WHO on the ground to combat the disease.

Rodents and rains

The plague is caused by Yersinia pestis, a bacteria found in rodents and spread by fleas.

Recent flooding in the nation has displaced tens of thousands of people and an "untold numbers of rats," leading to fears the disease could spread, said Margaret Chan, director-general of the World Health Organization.

How is it spread?

Once an infected flea bites human beings, they can develop the bubonic plague, which is marked by swollen lymph nodes.

If the bacteria reaches the lungs, one can develop pneumonic plague.

The pneumonic type is rare but more dangerous than bubonic plague because it can be transmitted between humans through inhalation and coughing.

"If diagnosed early, bubonic plague can be successfully treated with antibiotics," the World Health Organization said. "Pneumonic plague, on the other hand, is one of the most deadly infectious diseases; patients can die 24 hours after infection."

At least 8% of cases advance to pneumonic plague, the WHO said. It's unclear what percentage of the current cases comprise the more lethal plague.

Past plague epidemics have have occurred in Europe, the United States , Africa, Asia and South America.

The plague was known as the "Black Death" in Europe in the 14th century, and led to the deaths of 50 million people, the WHO said.