Escherichia coli (E. coli) bacteria normally live in the intestines of people and animals. Most E. coli are harmless and actually are an important part of a healthy human intestinal tract. However, some E. coli are pathogenic, meaning they can cause illness, either diarrhea or illness outside of the intestinal tract. Pathogenic E. coli can cause urinary tract and bladder infections, or lead to sepsis. Septic arthritis due to E. coli can also occur usually in patients with underlying systemic disorders.