Some patients with coronavirus are taken to intensive care units (ICU) to be given some extra help to combat the disease, with Boris Johnson becoming the most high-profile figure to do just that.
He was initially admitted to hospital on Sunday night, and said himself the following afternoon that he was "in good spirits" - but by evening his condition had worsened and he was moved to an ICU.
A Downing Street spokesman said the move had been one of "precaution should he require ventilation", and that the UK prime minister was still conscious.