<s>[INST] How does a kidney infection start? [/INST] Bacteria that have infected the bladder may travel up the ureters to the kidneys. This can cause pyelonephritis, a kidney infection. An infection in the upper tract may cause a more severe illness than infection in the lower tract. Source: ACOG. Link: https://www.acog.org/womens-health/faqs/urinary-tract-infections. Field: Urology. Disease: Urinary Tract Infection  </s>