Intensive care is where doctors look after the sickest patients - his admission to ICU is the clearest indication of how ill the Prime Minister is.
We do not know the full details of Mr Johnson's condition, but he is conscious and not being ventilated.
Not every patient in intensive care is ventilated, but around two-thirds are within 24 hours of admission with Covid-19.
This is a disease that attacks the lungs and can cause pneumonia and difficulty in breathing.
This leaves the body struggling to get enough oxygen into the blood and to the body's vital organs.
There is no proven drug treatment for Covid-19, although there are many experimental candidates.
But the cornerstone of the PM's care will depend on getting enough oxygen into his body and supporting his other organs while his immune system fights the virus. BBC