Although much work is done by food authorities in the event of a foodborne illness outbreak to identify the source, there are occasionally times when that work proves fruitless.
Knowing the source helps prevent further potential outbreaks from there.
The Center for Disease Control (CDC) in the USA has just declared that a third E. coli outbreak in the last year is over without a cause being identified.
In this outbreak 22 cases were identified across seven US states, with 11 people going to hospital and one person has died.
Whole genome sequencing has identified that all of the cases had closely related strains of E. Coli O157:H7, so it was from the same cause, but even with thorough investigation a common source has not been found.
Three of the cases developed hemolytic uremic syndrome (HUS), a type of kidney failure.
The first unknown source outbreak of E. coli had 40 cases across 19 states and 20 of those people were hospitalised. It was linked to leafy greens but no definitive source was identified.
The second unidentified outbreak had 32 cases across 12 states and 15 needed hospitalisation.
E.coli symptoms include severe stomach cramps and diarrhea, which are often bloody, and possible fever. Although this pathogen can cause long term and life threatening complications, most people recover within five to seven days.
According to the CDC around five to 10 percent of those with a E. coli infection can develop HUS, which includes symptoms of fever, abdominal pain, feeling very tired, decreased frequency of urination, small unexplained bruises or bleeding, and pallor.
Most will recover in a few weeks from HUS but in some death is a potential result. It is the High Risk groups which are most susceptible to HUS.