just because you're exposed doesn't necessarily mean you'll get it either
we all carry loads of potentially dangerous micro-organisms
and loads of "beneficial" micro-organisms
it's all nicely balanced to maintain our health
you could probably be exposed to salmonella most days and your immune system could deal with it
then one day, you're a bit run down, a couple of stress factors up the ante, you eat the wrong thing - and the next day you're struck with the runs
gosh - most of us have that scenario every day - it's amazing we're not sick all the time