By this time George's health was deteriorating. He had a mental illness, characterised by acute mania, which was possibly a symptom of the genetic disease porphyria, although this has been questioned. A study of samples of the King's hair published in 2005 revealed high levels of arsenic, a possible trigger for the disease. The source of the arsenic is not known, but it could have been a component of medicines or cosmetics. The King may have had a brief episode of disease in 1765, but a longer episode began in the summer of 1788. At the end of the parliamentary session, he went to Cheltenham Spa to recuperate. It was the furthest he had ever been from London—just short of 100 miles (150 km)—but his condition worsened. In November he became seriously deranged, sometimes speaking for many hours without pause, causing him to foam at the mouth and making his voice hoarse. George would frequently repeat himself, and write sentences with over 400 words at a time, as well as his vocabulary becoming more complex, possible symptoms of bipolar disorder. His doctors were largely at a loss to explain his illness, and spurious stories about his condition spread, such as the claim that he shook hands with a tree in the mistaken belief that it was the King of Prussia. Treatment for mental illness was primitive by modern standards, and the King's doctors, who included Francis Willis, treated the King by forcibly restraining him until he was calm, or applying caustic poultices to draw out "evil humours".