The E.coli outbreak sweeping Europe is the deadliest on record: 22 people have died and there have been over 2,000 reported cases in nine countries.

Ordinarily, E.coli is a harmless bacteria found in the gut. They are found in every other mammal, too. Just like us, the bacteria have sex and get old. They send out assassins. They form biofilms that resemble cities.

But in the mid-1900s, scientists began uncovering strains of E.coli that could cause life-threatening diarrhea. Unlike ordinary E.coli, they carried genes for a poison known as Shiga toxin, named for Japanese bacteriologist Kiyoshi Shiga. Over time, microbiologists identified a number of strains of disease-causing bacteria, classifying them by the proteins on their surface.

It is one of these strains that have caused the current outbreak in Germany. It enters the food chain via contamination with faecal material.

As health official struggle to trace the outbreak to a food that can be removed from the market, it has focused international attention on the complex paths that have led to the emergence of this rare strain of the bacteria.