How could the River Thames become so polluted so quickly?

Fifty years ago, the river was so polluted that it was declared biologically dead. In the 19th century people regularly contracted cholera from the water and died.

map scale refers to the relationship (or ratio) between distance on a map and the corresponding distance on the ground. for example, on a 1: 100000 scale map, 1cm on the map equals 1km on the ground.