If I take anti malaria pills, how likely is it to contract the disease?

i'm going to kenya and i'm taking anti malarial drugs but how likely is it that a contract malaria even with taking these pills??? cheryl cole took them and still got it, and i know it's possible anyway. and how likely is it that i get it and it stays dormant until i leave and stop taking the drugs? also i went to another country with equal high risk of getting malaria and i took the drugs and i got bitten a lot by mosquitos even with all the precautions and i never got malaria, so it can't be that likely, right?