THAT'S THE WHY:It's a tiny bug but it has a big, bad name: Escherichia coli, or E.coli for short. It has a sullied reputation for causing gastrointestinal upsets that can threaten life in severe cases.

But before we look at why this bug wreaks havoc, let’s not tar all E.coli bacteria with the same brush.

There are over 700 “serotypes” of the bacterium and some of them live harmlessly in your gut. In fact they can contribute to good health by making vitamin K and crowding disease- causing organisms out.

Plus, some strains really shine as workhorses in biological research, and scientists use E.coli cells in the lab to develop genetic tools and constructs that help us understand how cells work and how disease arises.

So E.coli is not always the villain of the piece. But one variant in particular has given the species a rotten reputation: E.coli 0157:H7.

If we ingest this bad apple through contaminated sources like water, undercooked meat, unpasteurised juices or milk or unwashed vegetables, it can cause illness by releasing verotoxin, which kills cells that line small blood vessels.

That can result in symptoms of vomiting, abdominal pain and bloody diarrhoea. A proportion of very young and elderly people may go on to have even more severe complications, including kidney failure.

Only a few live cells of toxin-producing E.coli are needed initially to cause illness, but good hand hygiene, washing raw vegetables and thoroughly cooking meat can help avoid regrettable after-effects of that impromptu barbecue.