Excessive sickness is a very tricky problem.
We all know that some employees take the p*ss regarding sickness. Getting drunk the night before, Christmas, bank holidays, Sundays, child care issues, a Cup Final match, wanting to avoid a certain Captain, or SIM or TRE. We can probably all think of someone who has done this.
Any HR department worth their salt would have stats of the normal rates of sickness in the working population - probably specifically within aircrews, maybe even within fleets - and so be able to flag-up possible cases of dishonest sickness.
Mrs Uplinker tells me that when she was with a UK flag carrier; she heard of cabin crew who claimed pregnancy and faked their pregnancy tests.
On the other hand, some people are just unlucky with their health. Some companies have a return from sickness interview to make sure the returning employee is OK, and in the case of repeat "offenders"; whether there are any lifestyle changes that might be beneficial to their health. e.g. sleeping, diet, exercise, vitamins etc.