Answer: Milo of Crotona progressively increased his workload by starting at lifting a baby calf and keeping on lifting the calf as it grew bigger.
The answer is urine because it is a liquid waste that filters blood.
I would say that <u>a deadly water-borne microorganism in a city's water source</u> would most quickly devastate a human population, merely because a huge number of people uses this water for some kind of a purpose.