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, yesterday.
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 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 at home, or if they need to go out, to wear a mask so they do not spread the disease to others.
Professor Tan Chorh Chuan, chief health scientist at the Ministry of Health, said: "In today's environment, be prudent. If you are feeling febrile, it's better to stay at home."
They were speaking at a press conference on how the local public health and research and development communities are working together to combat the spread of the coronavirus.
Prof Leo said much is still not known about the disease, why some people get mildly ill while others may die of it.
She added: "I will say that this is not really a mild disease, from what we can observe so far, having a few patients in the intensive care unit (ICU) at this point in time."
From global cases, she said it appears that the elderly, people with underlying health problems or those who are immuno-compromised tend to fare worse.
In Singapore, several patients have needed either high-dependency care because their condition is unstable, or even intensive care as they require intubation to ensure they get sufficient oxygen.
As of Sunday night, six patients were in intensive care. These are patients of different ages and not just the older ones, Prof Leo said.
UNDERSTANDING THE VIRUS
To fight a war, you must know your enemy. This is a new virus, you must know what you're dealing with.
MINISTRY OF HEALTH'S CHIEF HEALTH SCIENTIST TAN CHORH CHUAN
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 rates, blood pressure and temperature. Any change triggers an alert and medical staff will attend to 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 work 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 lungs, 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.