Is my immunity going down?

It seems for the first time in years, I've been sick..not once, but twice. As I type this I have a mild cough, kind of a runny nose, and my voice is half gone. In October, I had the flu. I'm 31 years old, and before this I rarely, if at all got sick. Is this happening because I'm gettin' older, or has this just been a particulary bad flu season?