The second time you are exposed to a specific pathogen, you have a faster, less severe illness. this is because:

This is because you have had it before therefore your body knows how to get rid of it,you have also become more amuned to it. 
we are able to get rid of a certain illness sickness or even pathogen  faster the second time you get it due to the fact our bodies have had it before which mean they have experienced how to get rid of it

An advantage of an mri over ct scan is d. it produces better images of hard tissues. hope this : )