Commonly known viruses include Rotavirus, Hepatitis A, Norwalk, and Polio. 1990.Abram S. Benenson, editor. American Public Health Association. The vast majority of pathogens are introduced by animals, but human beings are also carriers. Feces wash into drainages, carrying with them the disease-causing pathogens.
Water moving through the drainages spreads pathogens downstream.
Reactions range from no symptoms at all, to desperate illness, to death for some immune-compromised individuals. Your reaction will depend on the pathogen and on your immune system. Immune systems vary from individual to individual, based on genetic make-up and medical history. You may be one of the lucky few who experiences few, mild, or no symptoms. For the rest of us, after drinking contaminated water, there will be an incubation period of between a few hours and a couple of months, during which no symptoms will be evident.