It seems that higher population would automatically imply more contact with others so the chance of spreading disease goes up. But I have also thought for some time that as population density goes up, mental health goes down as well. Not sure the cause/effect, just my opinion.

It’s too broad of a question or theory. A lot depends on combinations of the following:

- location (cold climate, warm climate)
– educational level of population
– availability of medical care
– age distribution of the population
– economic viability

There isn’t one single answer. Too small of population density has its own dangers. To dense a population has its problems too,