If i find fleas on my cat will a spot on treatment completly banish them?

i put a flea collar on and combed the cat till no more were present, i then left it a few days and found more fleas, i washd the cats bed and blankets at 60 dgrees to kill em off there so why am i still finding fleas on my cat? its a 12 week old female un spayed so wont be going out side yet