Cholera is a terrible and painful disease. https://health.google.com/health/ref/Cholera
It hasn't been found in Haiti for many years. The Nepalese army is being investigated as the source of the outbreak. They might have inadvertently contaminated a major water source with their waste products.
Without proper sanitation, like clean water and soap to wash hands the disease will spread. There are no public water works on the island.
Cholera clinics are being burned down by worried residents.
NPR has comprehensive coverage of the outbreak.
A great article by Ezili Danto
Democracy Now coverage of cholera outbreak
It's very sad to watch the people of Haiti suffer so much. It's event after awful event striking an already stressed population. There was so much aid promised after the earthquake and yet there has been no change in the condition of the people in terms of jobs, housing, removal of rubble, food and water security.