SINGAPORE - Several of the locally infected patients with no known links had seen a doctor two or three times before they were sent to hospital.
This is because the illness generally starts mild with non-specific symptoms such as cough, sore throat, slight fever and feeling tired, said Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases (NCID).
These are symptoms tens of thousands of people suffer from every day in Singapore alone, she added.
So it is difficult to identify those infected by the novel coronavirus if they have no links to China or to other patients.
She said they may be mildly sick for more than a week before the virus affects their lungs.
That is why those who feel unwell are advised to stay home, or if they need to go out, to wear a mask so they do not spread disease to others.
Professor Tan Chorh Chuan, chief health scientist at the Ministry of Health (MOH), said: "In today's environment, be prudent. If you are feeling febrile, it's better to stay at home."
Prof Leo said that much is still not known about the disease; why some people get mildly ill, while others may die of it.
From global cases, she said it appears that the elderly, people with underlying health problems or those who are immuno-compromised tend to do worse.
In Singapore, several patients have needed either high dependency because their conditions are unstable, or even intensive care as they require intubation to ensure they get sufficient oxygen.
As of Sunday (Feb 9) night, six patients are in intensive care. These are patients of different ages, and not just the older ones, she said.
The first six patients who recovered all had mild symptoms.
Patients in intensive care whose lungs are no longer working well get mechanical aid in their breathing.
Patients in both ICU and high-dependency wards are continuously monitored - that is, minute by minute - for their oxygen level, heart and respiratory rate, blood pressure and temperature.
Any change triggers an alert and medical staff will be with them immediately.
Prof Tan said some patients "mount a strong immune response to the virus".
It is sometimes this strong response that does harm to their bodies, occasionally damaging their organs.
Prof Leo said the body "has a certain degree of redundancy, such as two kidneys, which most of the time works at 50 per cent".
When the person's immune response affects the organs, the doctors try to get the patient through the illness without too much damage to their organs.
As this virus affects primarily the lung, that is where most damage may occur.
She added that in the patients here, there has been little effect on their other organs so far.
"There are very few whose kidneys or livers are affected. It's mostly the lungs," she said.
A bit of good news - one ICU patient is now feeling better and came out of intensive care on Sunday, she said.