The human olfactory system is the product of millions of years of evolution, it's highly adept at detecting food that's no longer edible. Nature doesn't come with labels, that's why the smell of inedible food is so obvious and fowl.
> We carried out a survey of campylobacter in chicken on retail sale in the UK between May 2007 and September 2008, and it reported that campylobacter was present in 65% of the fresh chicken samples tested. A European Union baseline survey carried out in 2008, and published by European Food Safety Authority in March 2010, showed the UK estimated prevalence for campylobacter in broiler batches (caecal contents) was 75.3% and 86.3% in broiler carcasses (skin samples). These results were above the weighted EU mean prevalences of 71.2% and 77% respectively. There was a wide range of campylobacter prevalence across members states varying from 4.9% to 100% in broiler carcasses and from 2% to 100% in broiler batches.
For example an unpasteurized cheese has a whole ecosystem of bacteria, each keeping the others in check.
The cheese like substances in most American grocery stores however are almost sterile, so a single bacteria like Campylobacter can colonize it and take it over unopposed.
Not all bacterial contamination leaves a detectable smell.
