Intensive Care Units are specialist wards for patients who are too ill to be treated elsewhere.
They have sophisticated equipment that allows close monitoring of how the body is coping, as well as ventilators and oxygen. And there are highly skilled, specialist staff.
ITV News Health Correspondent Emily Morgan explains what they do, and what treatment the Prime Minister could receive.
In This Story: Boris Johnson
Boris Johnson has been Prime Minister of the United Kingdom and Leader of the Conservative Party since 2019.
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In This Story: Emily Morgan
Emily Morgan is Health Correspondent for ITV News. She was previously a political correspondent based in Westminster. Morgan has been a prominent reporter during the novel coronavirus pandemic of 2020.