Self isolation neighbor

Good day, I would like to know what the procedure is when boarding a plane from Cpt back to Jhb as I have received news that one of our family friends came back from Cpt and now the whole family is in self isolation. Firstly why was her temperature not checked before boarding? If they have a flu are they still allowed onto the plane? She was let onto the plane now how many people contracted the flu from her?. This is a serious issue as we are trying to minimize the spread but if airlines are not taking the necessary precautions regarding the health issues before allowing people to board the plane then why are we still traveling???. Please assist me with this as I am confused as to why this lady came back home and now is in self isolation. Regards A concerned citizen