An infection in any part of the urinary system (kidney, ureter, bladder or urethra).

UTIs, particularly by E. coli in women, are extremely common. Widespread use of antibiotics has led to high levels of drug-resistant bacteria that cause UTIs, many caused by bacteria carrying extended-spectrum beta-lactamases (ESBLs).

Complicated UTIs (cUTIs) are usually in people with underlying conditions, for example structural or functional abnormalities in the urinary system, other underlying health issues, or the impact of medical interventions such as use of catheters. Most UTIs in older people are cUTIs. cUTIs increase the risk of treatment failure, complications and antibiotic resistance.