It is undoubtedly true that organisations can track sickness rates as Jack has said. It is equally true that those working irregular hours and weekends are more prone to the kind of correlations to which Jack refers. It is also true that certain “spikes” in sickness rates are sometimes so remarkable as to strain belief.
However, note that Jack does not specify that he is talking about pilots. Some pilots, particularly younger pilots, may indeed show some tendency to the same syndrome. However, the available evidence suggests that of all the employee groups in any given company the pilots have by far the lowest incidence of such statistically interesting behaviour. (The general incidence of the syndrome may vary from company to company, but that is another subject).
The point is that for as long as pilots have a legal obligation to be fit for duty they must in the first instance be given the benefit of any doubt.
What we have started to do here is drift from the subject, which is the impact of the Ryanair culture on the willingness of pilots (and cabin crew for that matter) to turn up for duty in a fit state. The consensus seems to be that the impact of the Ryanair culture is not favourable to making such decisions. (Which, if you wish, can be taken to be a tactful way of say it is blindingly obvious to everyone except Ryanair management, the CAA, the IAA and some PPRuNe posters).