Frequent flu and cold. What should i do?

I am 30 year old i am noticing that i got infected frequently by flu, influenza and other minor cold problems. Presently 2nd time in a month i am facing, Not severe but 3-4 days cold, throat choke and little nastoral block, cough and generally i took steam for 3-4 days or dcold total pills to treat myself means majorly home remedial not doctor visit. Now i want to know why so frequently i am effected by these viruses and how can i avoid? Is there any immunity issue if any then what should i do? Is there any big issue Observation: sometimes i noticed that in sweating i drink cold water and sometimes whole day i drink less water and in the evening i get throat ache and nastoral block with little cough. Even i noticed when i came out from my office in evening the outside temp is so hot and i feel a sudden change in temp and feel uneasy. One thing is very strange my boss is always with me in same circumstances and he is 60 year old but he never got infected this thing creat fear.