A drugs test for athletes is 99% reliable: applied to a drug taker, it gives a positive result 99% of the time;given to a non-taker, it gives a negative result 99% of the time. It is estimated that 1% of athletes take drugs.What fraction of randomly tested athletes fail the test?

I am completely lost on how to do this as i wasn't in class that day so how would I use bayes theorem to solve this? 
Thanks