Yes, in the old days many disease epidemics were caused by cross-connections between potable water systems and raw river water or lake water piping systems. Epidemics of typhoid and cholera were often caused by backflow occurrences from these sources. People died or became very ill as a result of these outbreaks. A few of the contaminants caused by cross-connections are:
Untreated river, sea or lake water, pesticides, herbicides, fungicides, propane gas, worm treatment for poultry, boiler water with chemicals, anti-freeze, blood and body fluids from funeral homes, chemicals, water from car washes, dyes, sewage, Freon, worms, heavy metals such as arsenic, petrochemicals, water from flush toilets, bacteria cultures from laboratories and others.
This is only a partial list of documented cases of potable water contamination by virtue of cross-connections and backflow occurrences. They still happen, somewhere, every day.