Can someone please explain to me why a test for an extremely contagious illness takes longer to get the results from than the period of being contagious?

There's a confirmed case of Pertussis, aka Whooping Cough, at my school. The staff found out yesterday, and we all got Dept. of Health information sheets to educate ourselves about it and our students as well. According to this information, I have all of the symptons, and the fact that it is extremely contagious made me decide to go to the doctor and be tested. So I got a thin wire probe shoved up my nose and down my throat to "swab" for a culture. That just makes it sound so much more pleasant than it really was. And after this was done? I was told that the results would take 8-12 days to come in, and by then if I had it, I wouldn't be contagious anymore. So if I do have it, I get to go back to school and merrily spread it around.

Perhaps this is the reason why it's becoming much more common all of a sudden, hmmmm?