Why have i had cold/flu so much...?

so i usually get two colds a year, one in summer one in winter and are usually pretty mild. so this year i got one in may which was quite bad and this was the time i decided to quit smoking (as i didnt want to smoke when i my throat was sore) that lasted about 2 weeks, i got better and then i got another cold at the end of june, another harsh one, and recently like mid september i have been feeling congested again. im thinking my immune system should IMPROVE since i quit smoking why have i been more vunerable......it feels like i was stronger when i DID smoke...i just want to feel normal and strong again.