How long post SHTF before a city is safe to re enter?

[BDG]

It depends on what the disease is and how long people continue to survive there and act as a disease reservoir. A better idea and one which I expect anyone who survives long enough has is to scout in to the city over a period of months.

After all, you are not going to charge into a town or city center without knowing you have a way out - you may have safely got in through miles of suburbs on the only safe route - once dicey thing causing you to have to make a run for it any you could be way off course.

Also, unless you completely cut yourself off from people you are going to meet people who you may get news from, information, are refugees still coming out of the cities, how long have they not being coming out for etcetera.

Water borne diseases as mentioned can be overcome by boiling water, the bigger threat is people.