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A deadly outbreak of the plague — one of the most devastating pandemics in human history — is spreading in cities across Madagascar, the World Health Organization warned.

The disease has killed 21 people and infected 114 people since it was identified in late August. The country already has the highest number of cases of the disease in the world, but this time the plague is spreading to large urban areas, which increases the risk of transmission.

“WHO is concerned that plague could spread further because it is already present in several cities and this is the start of the epidemic season, which usually runs from September to April,” Dr. Charlotte Ndiaye with the WHO said in a statement.

“Our teams are on the ground in Madagascar providing technical guidance, conducting assessments, supporting disease surveillance, and engaging with communities,” she added. “We are doing everything we can to support the government’s efforts, including by co-ordinating health actors.”

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Officers from Ministry of Health spread pesticide against carriers of plague such as fleas in a primary school at Andraisoro, a district of the Antananarivo. RIJASOLO/AFP/Getty Images A woman wearing a face mask works in Antananrivo, Madagascar as plague spreads rapidly in cities across the country. Henitsoa Rafalia/Anadolu Agency/Getty Images Children wearing mask to protect themselves against possible infection of plague walk in Antananarivo. RIJASOLO/AFP/Getty Images A man wearing a face mask works in Antananrivo, Madagascar.

Bubonic and pneumonic plague

The outbreak in Madagascar includes two forms of the plague. Bubonic plague is spread by rats infected by flea bites and the pneumonic plague is spread by person-to-person transmission. Nearly half of the cases identified so far are of pneumonic plague, the WHO said.

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The plague is a disease of poverty and thrives in places with poor sanitary conditions and inadequate health services. It can kill quickly if left untreated, but can also be cured by common antibiotics if delivered early, according to the WHO.

The deadly disease is endemic to Madagascar, which is an island nation off the southeast coast of Africa. There are usually 400 cases — mostly bubonic — reported annually. But this year the numbers are higher than usual.

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Historically, the plague was responsible for widespread pandemics with high mortality. It was known as the “Black Death” during the 14th century, causing more than 50 million deaths in Europe. The plague is easily treated with antibiotics and the use of standard precautions to prevent acquiring the infection.

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The plague in U.S., Canada

Although the plague is typically a disease that thrives in low-income nations, the United States and Canada have had cases over the years.

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According to the Public Health Agency of Canada, there are around 10 to 15 cases of the disease every year in the southwestern U.S. In 1996, there were two deaths attributed to the plague in the U.S.

Human cases of plague are very rare in Canada with the last case reported in 1939.