If you are asking someone to babysit for you isn't it common sense and common courtesy to at least warn the babysitter that the child is sick before you bring them?

Most definitely. 

Worst case:  They have no idea if the sitter or a member of his/her family has a compromised immune system. 

Even a healthy person has the right to know in advance of exposure.  Others don't want the extra responsibility.

I personally would probably say yes, but I'd like to know what I'm in for.

I think that is a very reasonable courtesy.

Yes. But then again things that would seem common sense, I realize really aren't always common.

I wouldn't babysit. Too easy for all to get re-sick or partially re-sick again.

Yes it would be a matter of common courtesy .. No question about it

At 11:30 pm last night, my daughter got a text from our neighbor asking if she wanted to babysit her one year old son from 4am till noon. My daughter asked for 5 dollars an hour instead of the usual 25 dollars total for the whole babysitting job cuz it was last minute and we had stuff to do today but she didn't want to leave the neighbor without a sitter. Our neighbor offered to bring him at 6am instead till noon. At 7 I woke up to take my 9 year old to school and my daughter says mom I think the baby is sick. I said "WHAT!?! I JUST got everyone better in the house!" I advised her to call the mom and ask if he is sick and she said "oh yes he has the flu." I know I am old but has manners seriously gone out the door now a days or was the mom just THAT desperate!?!