On the historic Oregon Trail followed by pioneers, cholera was caused by the bacterium Vibrio cholera and spread primarily due to poor sanitation. Contaminated drinking water was often the source of the bacteria.
Cholera had spread to the New World from the Indian subcontinent, where it was an epidemic. By the time of the westward expansion, the bacteria was common in American waters. During spring flood conditions, when waters were warm and phytoplankton common, Vibrio cholera multiplied rapidly, often infecting settlers who drank unboiled water or crossed contaminated rivers. While only a small percentage of people infected with the cholera virus became ill, the sick were often doomed to die quickly of dehydration. Symptoms included severe vomiting and diarrhea.