When the bacteria get out of the intestinal tract and into the urinary tract, they can cause an infection sometimes referred to as "honeymoon cystitis" because intercourse can lead to introduction of bacteria into the bladder. Although it is more common in females, urinary tract infection is seen in both males and females. It is found in roughly equal proportions in elderly men and women. Since bacteria invariably enter the urinary tract through the urethra, poor toilet hygiene can predispose to infection but other factors are also important (pregnancy in women, prostate enlargement in men) and in many cases the initiating event is unclear. Anal sex is also a risk factor for males.
When the bacteria get out of the intestinal tract through a perforation (a hole or tear, which could be caused by an ulcer, for example) and into the abdomen, they usually cause an infection called "peritonitis" that can be fatal. However, E. coli are extremely sensitive to antibiotics such as streptomycin or gentamycin, so treatment with antibiotics is usually effective.
Certain strains of E.coli are toxigenic (some produce a toxin very similar to that seen in dysentery) and can cause food-poisoning usually associated with eating contaminated meat (contaminated during or shortly after slaughter or during storage or display). Severity of the illness varies considerably; it can be fatal, particularly to young children, the elderly or the immunocompromised, but is more often mild. E. coli can harbor both heat-stable and heat-labile enterotoxins. The latter, termed LT, is highly similar in structure and function to Cholera toxin. It contains one 'A' subunit and five 'B' subunits arranged into one holotoxin. The B subunits assist in adherence and entry of the toxin into host intestinal cells, where the A subunit is cleaved and stimulates the cells to release water, causing diarrhea.

Strains
 
Model of successive binary fission in E. coliA "strain" of E. coli is a family with some particular characteristics that make it recognizable from other E. coli strains, the way poodles are recognizable from other strains (or "breeds") of dogs, and different strains of E. coli live in different kinds of animals, so it is possible to tell whether fecal material in water came from humans or from birds, for example. New strains of E. coli arise all the time from the natural biological process of mutation, and some of those strains have characteristics that can be harmful to a host animal. Although in most healthy adult humans such a strain would probably cause no more than a bout of diarrhea, and might produce no symptom at all, in young children, or in people who are or have recently been sick, or in people taking certain medications, an unfamiliar strain can cause serious illness and even death. A particularly virulent example of such a strain of E. coli is E.coli O157:H7.

Role in microbiology
Because of its ubiquity, E. coli is frequently studied in microbiology and is the current "workhorse" in molecular biology. Its structure is clear, and it makes for an excellent target for novice and intermediate students of the life sciences.