Complex sentence: The nineteenth century had been marked by periodic outbreaks of epidemics of cholera and other infectious diseases, often brought by travelers on the major river systems and transmitted through water contaminated by the poor sanitation of the time.
Split into simpler sentences: The nineteenth century had been marked by periodic outbreaks of epidemics of cholera and other infectious diseases, often brought by travelers on the major river systems. It was transmitted through water contaminated by the poor sanitation of the time.