Where E. coli comes from and how it kills

The E. coli bacteria occurs naturally in the guts of most warm-blooded animals. All humans carry the common forms of E. coli in their intestines. It's essential for the healthy functioning of digestive systems, helping humans synthesize vitamins and suppress the growth of harmful bacteria.

But not all forms are so helpful. E. coli O157:H7, which was first identified in 1982, is a particularly nasty version. It secretes a powerful poison, called a verotoxin, that binds to receptors on human kidney, brain and gut cells and kills them.

Not all people have the receptors, which explains why some people — and animals, including cattle — who get O157:H7 become very ill and some don't.

E. coli O157:H7 is generally transmitted via infected fecal matter that has contaminated soil, water, fertilizer or, in the case of tainted meat, hamburger.

The danger to those infected with O157:H7 is that they will get a kind of kidney failure called hemolytic uremic syndrome, or HUS. It hits the very young and the very old the hardest.

People severely stricken with O157:H7 say it feels like a million tiny knives going through the body. Within days of symptom onset, what began as diarrhea turns to blood as the intestines break down.

By Elizabeth Weise, USA TODAY