There is evidence that the new coronavirus can be spread before the infected person shows any symptoms.
This, however, is uncommon, said Health Minister Gan Kim Yong, as he spelt out what is known so far about the virus. He said this form of transmission has so far involved isolated cases only.
At this point, the evidence still points towards higher transmissibility when the person is displaying symptoms - which is generally the case with coronaviruses, he told Parliament yesterday.
Those who are exposed to the virus may be well for a few days before developing symptoms such as a fever or cough. Some may subsequently develop pneumonia.
The coronavirus was first reported in the city of Wuhan, in China, on New Year's Eve. As of yesterday, it has affected more than 17,000 people and killed over 360 in China. There are now 18 confirmed cases in Singapore - 16 Chinese nationals and two Singaporeans - all of whom had travelled from Wuhan.
Although there is now no evidence of community spread in Singapore, people here and in some other countries have been scrambling to stock up on masks to protect against the coronavirus.
Currently, the evidence suggests that transmission is mostly via droplets. "What this means is that the virus is carried within droplets emitted from an infected person over a short distance, such as when the person coughs or sneezes," Mr Gan said in a ministerial statement.
"If these droplets come into contact with the eyes, nose or mouth of an individual, directly or indirectly through hands that have come into contact with these droplets, the individual may become infected."
However, potential infection from asymptomatic persons is less likely to be from coughing or sneezing directly, but "more likely by touching contaminated surfaces, for which masks offer no protection", said Mr Gan.
Contact transmission can happen when a person sneezes or coughs, and the droplets fall onto the surfaces of tables and chairs, for example, where the virus may remain active for "most likely two to three days", although this is not certain yet, said Mr Gan.
Then, when someone touches the surface, the virus can be transferred to his hand, and if he then rubs his eyes or nose without washing his hands, he may become infected.
"This is also why we only quarantine the close contacts of confirmed cases. For more transient contacts, such as individuals that the confirmed cases may have walked past in malls or hotels, the risk of transmission is low."
A key reason why some people have started wearing masks is the fear of breathing in the virus in public. However, the minister said there is currently no evidence to suggest that the virus is airborne.
"There are other viruses, such as chickenpox, which can easily be transported via air currents and do not require droplets to contact the eyes, nose or mouth of another individual. The novel coronavirus is not in this category of viruses," explained Mr Gan.
Medical professionals continue to advise that the most effective protection is to practise good personal hygiene. Regular hand washing with soap and water, and not touching our faces with our hands, is a seemingly simple move that is actually very effective in preventing all kinds of infections, Mr Gan said.
"Wearing a mask when we are well often gives us a false sense of security instead, and we are more likely to touch our faces when we constantly adjust our masks, which is one way the disease spreads."
A light-hearted moment in Parliament yesterday came when Mr Gan relayed a message from Foreign Minister Vivian Balakrishnan. "He said while I was giving my speech, he has observed 13 out of 41 MPs sitting opposite us have touched their face, within just 25 minutes.
"So... touching (your) face or whatever it is, I think it is not easy to avoid. But please wash your hands."