It's emerging that many people infected with coronavirus may appear to have no symptoms at all. This has been referred to recently as the "Achilles Heel" of the public health efforts.
It is not unusual for infectious diseases to be spread by carriers who appear to be completely unaffected and there are many famous cases of this in the history of public health. Infections such as Typhoid fever, Cholera, HIV, Chlamydia, Tuberculosis and Poliomyelitis are all known to have been spread by people who display no symptoms at some stage of the disease. For Typhoid, it is estimated that between 1 and 6 % of people who have been infected become long term carriers.
This 'hidden infection' is one of the reasons why widespread testing has proved to be a very effective in limiting the spread of the Coronavirus infection. Countries such as Singapore refined their track and trace methods, learning from the precedent of the SARS epidemic of 2003. This has helped them to be amongst the most successful nations in countering Coronavirus.
There's an interesting and informative article in the New Scientist, here