They live inside all of us and usually pose no danger.
But one strain of the common Escherichia coli bacteria is very bad news. It creates a toxin that can shut down the kidneys and lead to death.
Humans can pick up the dangerous version -- labeled O157:H7 -- from contaminated food and beverages, or through contact with the manure of farm animals that sometimes harbor the germs.
Once exposed, people can suffer a range of effects. Some hardly get sick at all, or struggle for a few days with diarrhea or vomiting. Others fall very ill. Some die.
The Centers for Disease Control and Prevention estimates more than 70,000 Americans get sick with E. coli annually, and about 60 people succumb to it.
The bacteria also might trigger a dangerous kidney complication called hemolytic uremic syndrome, especially among children and the elderly. This is the illness afflicting a growing number of Central Florida patients.
In those cases, a few days after exposure to the E. coli, a person will have diarrhea and abdominal cramps. Some also will suffer from vomiting and fever.
These early effects often pass in two or three days. Even so, the patient won't feel any better. Children may be irritable and tired. They may stop urinating as frequently and bruise easily.
This is because the second phase of the attack has begun. The bacteria, which already may have been shed from the body by diarrhea, have left behind a toxin in the bloodstream.
This chemical poison causes microscopic clots that often lodge inside the maze of tiny vessels that feed the kidneys, said Dr. Michael Muszynski, an infectious-diseases expert and professor with the Florida State University College of Medicine.
As the blood tries to flow through the kidney's vessels, it gets snagged on the clots. "It's like a mechanical breakup up of the cells," Muszynski said.
The kidneys start to lose function, eventually putting the patient in life-threatening kidney failure.
Muszynski said there is no specific treatment for the condition, though doctors can relieve its effects with dialysis, blood transfusions and intravenous fluids.
From 10 percent to 30 percent of patients will suffer permanent kidney damage as a result, according to the National Institutes of Health. Some may need ongoing dialysis. Others might need a kidney transplant eventually.
Robyn Shelton can be reached at firstname.lastname@example.org or 407-420-5487.