WHY DIDN'T LEVI JOHNSTON CONTRACT MONO FROM BRISTOL PALIN?

I AM NOT A CONSPIRACY THEORIST OR ANYTHING, BUT I DO HAVE A SIMPLE QUESTION. WE DO KNOW THAT BRISTOL PALIN MISSED THE FINAL 5 MONTHS OF HER JUNIOR YEAR BATTLING MONONUCLEOSIS. IF SHE IS INDEED CURRENTLY 5 MONTHS PREGNANT, THAT MEANS SHE GOT "KNOCKED-UP" SOMEWHERE DURING THE TIME SHE SHOULD OF BEEN IN BED RESTING. BUT MY BIGGER QUESTION IS WHY DIDN'T HER BOYFRIEND LEVI JOHNSTON CONTRACT THE ILLNESS AS WELL? LEVI HAD TO KNOW THAT MONO WAS TRANSMITTED THROUGH THE EXCHANGE OF BODILY FLUIDS RIGHT? THE CONCEPTION PROBABLY WASN'T THE ONLY TIME THEY WE INTIMATE DURING THE 5 MONTHS EITHER. I NEED A COUPLE THINGS ANSWERED, WHY WAS BRISTOL TOO TIRED FOR SCHOOL BUT NOT TIRED FOR ? AND WHY WASN'T LEVI JOHNSTON AFFECTED?