Story highlights 165 people have died and more than 2,000 infected with plague in Madagascar

Since mid-October, the number of new cases has been on the decline

(CNN) The plague outbreak in Madagascar is continuing to slow, but nine countries neighboring the island nation are on alert in case the infection reaches their borders.

Since the outbreak began in August, 165 people have died and more than 2,000 have become infected with plague as of Wednesday.

Of those infections, 77% were clinically diagnosed as pneumonic -- or pulmonary -- plague, a form of the infection that can spread from person to person.

Plague is caused by the bacterium Yersinia pestis and is typically spread through the bite of infected fleas, which are frequently carried by rats, causing bubonic plague.

Pneumonic plague is more virulent or damaging and is an advanced form characterized by a severe lung infection. The infection can be transmitted via airborne droplets from coughs and sneezes.

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