It hasn't been answered a 100 times but we have discussed it before. The thing is MS is a problem of an OVERACTIVE immune system and many of us go yearsbetween getting sick, That said, I am at day ten with a case if cough/bronchitis crap thanks to my husband. I can't quite seem to shake this one, but it did take him about two weeks to feel better, so there is hope for me.
I agree with Laura that it's more likely the other direction, with an over-active immune system potentially making those with MS less prone. Though, one thing that can throw this theory at of whack is what treatment someone is on.
Most disease-modifying drugs are immunomodulators and don't actually suppress the immune system (the orals and CRABs injections), but corticosteroids (used for acute relapses) and some of the next generation treatments (like the monoclonal antibodies) are actually immuno-supressants. In this case, someone with MS may switch to the other camp and be a bit more prone to viruses, and opportunistic infections. I know I get my flu shot!