An outbreak of E. coli infections at several Calgary daycares has infected more than 260 people, most of them children. It's not the first major outbreak of the food-borne illness, which is caused by bacteria that live in the feces of animals and is carried to humans through undercooked meat, unpasteurized food or contaminated water. The E. coli variant known as 0157 is often the most virulent and can cause hemolytic uremic syndrome, which affects the kidneys and the ability of blood to clot.