The decision to move a coronavirus patient to intensive care is never taken lightly and is a clear indication that they need artificial support to help their bodies see off the virus.
A modern intensive care unit provides a wide range of life support systems, of which oxygen and mechanical ventilation are just two options.
It was reported today that British Prime Minister Boris Johnson was not, initially at least, placed on a mechanical ventilator.
More likely he is having his vital signs carefully monitored while receiving oxygen piped through a mask.
While mechanically ventilated patients are normally unconscious, oxygen treatment does not require this.
It is important to note that intensive care is not a "cure" for the coronavirus.
In the absence of specific drugs that fight the illness, it provides what doctors call "supportive care".
In severe cases, the virus attacks the lungs and causes viral pneumonia. This can progress very quickly and catch both patients and doctors by surprise.
It seems improbable that doctors looking after the Prime Minister would not have anticipated this as a possibility in advance - and it may be he has simply been moved to intensive care as a precaution.
Dr Bharat Pankhania, senior clinical lecturer at the University of Exeter Medical School, told the Telegraph: "It is difficult to know. As a Prime Minister, everyone will be on edge, they will be ultra precautionary."
In severe cases, oxygen and later ventilation, if necessary, effectively take the pressure off the patient's vital systems, giving them a better chance of fighting back.
In the most serious cases, coronavirus patients have an overly powerful immune response to the virus, which can cause severe inflammation and damage to the tissue of the lungs.
In otherwise healthy patients, it is this response - which doctors do not currently fully understand - that can be most dangerous.
Earlier this week, Professor Paul Hunter, professor of medicine at the University of East Anglia, pointed to early data gathered by the Intensive Care National Audit & Research Centre which suggests critical care for Covid-19 patients is less effective than for other types of viral pneumonia.
"Sadly about half of cases (50.1 per cent) that go into critical care [with Covid-19] still die. This is much higher than for other viral pneumonias (22.4 per cent). Why this is the case and how much higher this figure would be without critical care beds is not clear."
The chances rise to 54 per cent survival for someone Johnson's age.
Tom Solomon, director of the Institute of Infection and Global Health at the University of Liverpool said that some patients are moved to critical care beds simply for more intensive monitoring of their vital functions.
However, he added: "Most patients with Covid-19 who go to intensive care do so because of difficulty breathing, as a direct consequence of the virus's infection, or the body's immune defences which are trying to fight it."
At the most severe end a patient is made unconscious by drugs, and then a tube is inserted all the way into the lungs to do the breathing for the patient. Currently, we understand this is not being used for Johnson.
"At the mildest end, the patient is awake and just has some support with a tightly fitting mask, and higher pressure is applied to the airways through the mask, for example, CPAP, which is continuous positive airway pressure."
Professor Derek Hill, professor of medical imaging at University College London (UCL), said it was likely the Prime MInister was admitted to hospital after becoming breathless.
He said if his condition deteriorated further mechanical ventilation may be needed.
"Intensive Care Ventilators have sophisticated software and sensors to enable them to adjust the way they work around the needs of the patient, and to change the amount of oxygen inhaled with the air.
"One of the features of Covid-19 in all countries seems to be that many more men become seriously ill than women - especially in the over-40 age group.
"We also know that people under about 60 seem to have a higher chance of making a recovery from critical illness with Covid-19 than older people. The Prime Minister is 55.
Hill added: "It illustrates three of the important healthcare needs of Covid-19: Firstly many patients need help breathing, and there is a shortage of the mechanical ventilators that can do this - and in particular a shortage of the high-quality intensive care ventilators most suitable for Covid-19 patients who might need help breathing for over a week.
"Secondly, Covid-19 patients need a huge amount of oxygen to help them breath - which is potentially going to be in short supply.
"Thirdly, looking after people in intensive care requires skilled staff, and the experience of New York has been that finding enough skilled staff has been the greatest challenge."
Professor Linda Bauld at the University of Edinburgh, said the admission of the Prime Minister to intensive care was of "huge concern and illustrates just how indiscriminate this virus is".
She said: "Anyone, anywhere, including the most privileged in our society, can be affected and can become seriously ill.
"Questions will be asked in future about whether the UK Government acted appropriately in keeping Parliament open and face-to-face meetings going while the rest of the country was already following advice to shut down.
"For now, however, all our thoughts will be with the Prime Minister and his family, and the many other families who are facing similar circumstances with critically ill relatives."