Far to many tears to tye but here goes.
loudlass I'm very sorry for your loss.
sassyusan I have replied to many of your threads in the past and know how overwhelmed with grief you are for your beautiful daughter Catherine. I'm pleased you've found the strentgh to post here - I've certainly been very well edcuated. I also read your blog. You write beautifully and I was wondering if you've considered writing a book. I hope thats not an insensitive thing to say but I really feel you could educate people about bereavement.
My DS had chicken pox at 3.7. He had a spot and temperature so kept him home (well took him to dads as I work) and my dad took him to pharmacy who confirmed it was CP and gave calmomile. We didn't leave the house after for 5 days. That day when I picked him up from dads and got home neighbour invited me in I declined due to cp. She said to anyway as her DD was likely to have caught it as her and DS play daily. In hindsight maybe that was a very flippant attitude (she did get it exacly 14 days after DS). She was fine with it however DS was lifeless, extreme fever and a febrile convulsion which I called NHS direct about. He did get better but god above my heart bleeds for you sassy that you had to watch your DD go downhill further and lose her fight.
I lurked on the helmet thread and shabs you were amazing on this. My DS has never and will never go out without a helmet.
Yes life is full of risks, not sure anyone could reasonably argue otherwise, but to knowingly risk the lives of others.... 
OP, you are not being unreasonable to have asked or wondered about the risks, before this thread so would I have but unfortunatly a sad lessson has been learnt here tonight.