Diseases like dysentery, cholera, typhoid etc., are more common in over crowded human settlements. Why ?

$\Rightarrow$ Diseases like dysentery, cholera, typhoid etc., are more common in overcrowded human settlements with poor hygiene conditions.

Due to heaps of garbage, human excreta and animal excreta in open.

$\rightarrow$ because these are infectious diseases that can transmitted from one person to another.

Chances of spread of diseases like cholera, typhoid, $TB$ increases.

In overcrowded settlements there is more chances of transmission of disease from one person to other.