Answer: his health had been declining since 1818, when he visited Warm Springs, Virginia, to find relief for his rheumatism in the mineral baths.
Explanation: By some means, he contracted a nasty infection on his buttocks.2 This boil on his "seat" was probably more than furunculosis. Dr. Gordon Jones thinks it likely Jefferson developed a true septicemia, which would explain Jefferson's subsequent "prostration."