What's to explain? Every disease you listed can be found in nature and pollution isn't necessarily the cause.
Cholera. Caused by bacteria in water contaminated with feces (animal or human), stagnant water, or water that contains dead animals or fish. None of that requires pollution to occur.
Diarrhoea: Same cause as Cholera.
Dysentery: Same causes as Cholera
Hepatitis: Usually transmitted by a carrier to others. Again, nothing to do with pollution.
Typhoid: Usually transmitted from Salmonella bacteria from human feces or being unsanitary in daily practices.
Polio: Incubated in soil naturally and can affect entire populations. Most infections are non-debilitating and symptoms are mild. One of the major causes in the early 20th Century was increased cleanliness standards that caused more people, particularly the affluent, to not develop immunity at an early age.