dont worry the person involved wont be coming back, it was done before todayBut they do know why. They have a certificate from a qualified medical doctor who has certified that medically they are unfit to carry out the duties of work. The reason why they are medical unfit is not required, that is private.
Ask yourself, what are you going to do with the reason why they are sick. Are you going to judge the person? (Oh I had pneumonia and I came to work every day but this person just has a cold and their doctor is giving them two weeks, they are slacking... sort of thing.) The question the medical professional would ask is where are your medical qualifications to make such a judgement, it is not your place. What if you had a temporary staff member who was undergoing fertility treatment and you decided on the basis of this written in a medical certificate that you would not renew her contract even though her work was of a satisfactory standard? It is to prevent such scenarios happening that the reason for the illness does not need to be on the certificate.
What you have here is a performance related issue rather than an illness related issue. If you have an issue with the quality of their work then put them on a PIP. If you are concerned that they are fit for work despite being certified unfit by their doctor then send them to your company doctor. This is a cost to you but there is a process in place to do this. Companies like https://chi.ie/about-us/ are specialist in this sort of stuff (I have no connection to them). If your worker says they are too sick to drive there etc, then pay and send a taxi to collect them. Do it now, today, don't wait to let your suspicions fester.
And yes it is a performance issue but with a complicit doctor signing someone off.