What is the most heavily populated plain in south asia?

The correct answer is the Gangetic plain

It is also known as the Indo-Gangetic Plain, Indus-Ganga Plain and the North Indian River Plain, and is found on the north of India and is an alluvial plain in its structure. Because of the rivers and the land around it, it is a home to many cities and places with high population with huge numbers of urban areas with highly dense population.

The constant presence of diseases or infectious agents within a given geographic area or population group. it may also refer to the usual prevalence of a given disease with such area or group. it includes holoendemic and hyperendemic diseases. a holoendemic disease is one for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do children (malaria in many communities is a holoendemic disease). a hyperendemic disease is one that is constantly present at a high incidence and/or prevalence rate and affects all groups equally. so probably family history.