What Causes Frequent Bouts Of Fever, Cold And Chest Congestion?

Hello, this is Arif and i am 27. From the time i was 15 i get frequent fever cold and my nose, throat and chest get involved at least once in a month. sometimes it gets two three times a month. my nose and chest are always filled with phlegm (bulgum) especially in winters.i usaually take some amoxicilin to get rid of it. I have consulted many doctors. some of them said i have weak immune system and its due allergy from smells and cold. I also feel that because i feel lazy all the time. if that is the case what should i do. I have took chawanprash from dabur but no gain. can i take protinex suppliment