Answer: A deadly water-borne microorganism in a city's water source .
Devastation of human population refers to the severe and widespread damage or destruction of the population. The devastation can be because of natural or non-natural factors. Natural factors includes natural calamity like earthquakes, cyclones, landslides and volcanic eruptions, diseases and other factors. Non natural factors are fires, bomb explosion, nuclear explosion and other factors.
Water is the basic need of human population. Humans requires water for the purpose of drinking, bathing, cooking and other household needs. The uptake of microbe polluted water by human population can cause diseases which may be lethal.