I rarely get sick. is it likely that my immune system stops the flu and other colds rapidly or is it more likely that i am simply avoiding contact?

Could be both: There is definite variation as to the robustness of people's immune systems. You could be one of the fortunate that has an immune system that does a better job detecting and fighting pathogens. Another way to not get sick, as you mentioned, would be to not have contact with people in general.

Why?: Probsbly both. Living well helps the immune system stay strong and you may also be doing that.