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

[NorthernRaider]

(16 October 2012, 17:26)Ghost Wrote: SCENARIO.
What if I run out of arrows and the tools I had for making more have also broken? My only option is to enter the city and head for a shop that I know that may still have the tools to make more or maybe even some arrows that were over looked.
I am not entering the city out of choice but out of necessity. For this reason I need to know what precautions to take and how much possible danger from disease I may be facing.

Thats why I've got Cromwell tools, Machine Mart, Brown Bros etc marked on my maps, fortunately for us most industrial estates are often found towards the edge of cities so the risk is somewhat reduced, but not eliminated. After the Black plague ravaged Europe back whenever it took over 40 years for the last of the epidemic clusters to die out.