Feeling Sick from a Cold

I have a cold. I think I started to have it on Wednesday (due to the feeling I always have before I feel the symptoms). I didn't start to feel the symptoms until yesterday night, so it took me three days to feel them. I hope I won't end up with a fever or flu. I can't afford to waste my time for being sick. How did I get sick? This will be like the third time I get sick during this fall semester. Why do I get sick so easily now? How weak has my immune system gotten?