Oh you poor thing, I hear you! I am a teacher and suffer with the same thing in my classroom. I know that sometimes bugs are unpredictable or have long incubation periods but if your child is obviously ill then keep them home!
I am the same as you, what parents dont understand is that teachers/careers are under pressure to be at work and have families as well, my poor DD used to get it from both sides sick from daycare and sick from bugs I picked up at school- if everyone kept a sick child home for just a few days then everyone would be much better off!