How low can one child's immune system go?

Miss L is ill again.  I think her immune system had taken a complete battering this Autumn.  Since August she has had a two viruses, tonsillitis, a cold and now something else seems to be trying to take hold.  Her temperature is 39.  Illness always seems to strike a Monday, my study day when she is supposed to go to pre school.  I have an assignment due on Thursday so I am now panicking.  Why I am blogging I don't know, probably because I am actually getting quite worried about her.  It's just been one thing after the next. 

It has all become muddled too, she has definitely been tired and her energy levels aren't what they normally are.  So it feels like an uphill struggle all the time to get her doing stuff again.  I never quite know if she is genuinely tired or just trying to pull a fast one.  She admitted she had been pretending to be ill to get out of going to a swimming lesson.  'Just pretending Mummy ha ha.'  She loves being at home with me, especially as it means she gets my full attention while Mr G is at nursery.  This game playing is whole new territory for me.

You can't fake a temperature though (not at four years old anyway).  So today she stayed at home, and I tried to get my assignment done.  I struck a good balance between keeping a close eye on her, whilst making being at home as boring as possible.  At lunchtime her temperature got to 40, a place we have somehow managed to avoid for her four years and two months.  I got a bit scared, but calpol fixed it.  As I put her to bed she was muttering about wanting to go up in a hot air balloon and then bawling that Daddy had been in one (this was over a year ago) and not let her go in.  Very random.  But she had no temperature at this point (thank goodness for calpol) and she made complete sense after that (thank god - was starting to think she was delirious)

I am just hoping this means she will have a blinding immune system in the future.  Would love some perspective from more experienced parents.  My lovely dad reckons I take her to the doctors tomorrow, just to make myself feel better about it all.