well, I'm not sure you can ever really *know* when you were *first* exposed. For example you go to dinner with a friend. 5 days later she tells you she has the chicken pox. She was contagious 1-2 days before she broke out in the rash.
2 days later YOU break out in a rash. That would be 7 days after exposure.
IF you assume you got it from her.
What you DON'T know.... is that when you were at the grocery store 5 days BEFORE the party.... the kid who sat in the cart before you used it hadn't broken out in a rash yet, but was contagious and that kid slobbered all over the cart handle.
You touched the cart handle, at some point, and now you've been exposed.
which means you broke out in YOUR rash 10 days after you were exposed. see?
Also anyone who is immuno-compromised will have the infection worse and probably come down with it quicker.