Hah, 4 weeks.. my immune system was wiped from 7th grade through 11th grade! I'm a senior now, and just now getting any hint of an immune system!
The thing with Mono is that you will ALWAYS test positive with it. The only reason they knew I had Mono is that they did a test in 6th grade.. I was negative.. and 7th, I was positive. She said a lot of people are 'tricked' into having Mono because they test positive, when they could have had it when they were 5.
It definitely sucks though.. I had strep TWENTY times my 7th grade year!