Well, I just read that and the pertinent bit of Cap480Cs20 seem to me to be:
(8)
(a)
A person shall not be entitled to act as a member of the flight crew of an aircraft registered in Hong Kong if he knows or reasonably suspects that his physical or mental condition renders him temporarily or permanently unfit to perform such functions or to act in such capacity.
(b)
Every holder of a medical certificate issued under Article 19 or 20 of this Order who—
(i) suffers any personal injury involving incapacity to undertake his functions as a member of the flight crew; or
(ii) suffers any illness involving incapacity to undertake those functions throughout a period of 20 days or more; or
(iii) in the case of a woman, has reason to believe that she is pregnant:
shall inform the Chief Executive in writing of such injury,illness or pregnancy, as soon as possible in the case of injury or pregnancy, and as soon as the period of 20 days has elapsed in the case of illness
. The medical certificate shall be deemed to be suspended upon the occurrence of such injury or the elapse of such period of
illness or the confirmation of the pregnancy, and—
(aa) in the case of injury or illness the suspension shall cease upon the holder being medically examined under arrangements made by the Chief Executive and pronounced fit to resume his functions as a member of the flight crew or upon the Chief Executive exempting, subject to such conditions as he thinks fit, the holder from the requirement of a medical examination;
So firstly, the legal obligation seems to be to report an illness after it has rendered you unfit for 20 days, and secondly, let he who has never piloted whilst having a bit of a cough throw the first stone!