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

[Ghost]

It has often been mentioned that all cities will become a necropolis after TSHTF and it is for this reason that many of us are prepared to bug out. But it is likely that many of us will also need to scavenge these cities when our preps begin to dwindle.
My question is this.

How long should you wait before it is safe to re enter a city? How can you be sure that diseases like cholera and typhoid have run their course and died out?

I am of the impression that a year or at least a harsh winter will clear the city of disease. Am I right or wrong?