MIL told me and DH today that SIL has been taking methotrexate chemotherapy tablets for her psoriasis for about 7 years and this is why her immune system is fucked.
SIL is well known for being a total drama queen at the best of times, and if she has contact with one of DCs who has a little sniffle, then SIL will almost definitely contract full blown pneumonia and need to go to hospital, and be signed off work for at least 4 weeks. She works in a shop, and MIL is adamant that she is far too ill to be at work at all.
Every illness going, SIL develops, only with more/worse symptoms than anyone else.
For quite some time now, DH and I have been a bit
about how ill she actually is, and find it all a bit unbelievable tbh.
I googled the methotrexate and Wikipedia told me that one of the most common side effects of it is very dry skin and a sore rash - which sounds a bit like psoriasis to me 
It also may be relevant to point out here that until recently she worked in a school where she would have come into contact with lots of sick children.
I'm asking here as I know how much traffic goes through here, it's not really an AIBU, more looking for clarification I think......
There's a big back history with SIL, so I can't ask her directly.......
So what do you think??