The probability of infection from a virus is strongly dictated by the state of one's immune system.

“just because you come into contact with a colder flu does not necessarily mean that you will come down with that is be infected by a colder flu whether or not you come down with a colder flu is of course dependent on whether or not you come into contact with it you can't catch a cold or flu that you've never come into contact with just like you can't get eaten by a shark if you never go in the ocean however the probability of coming down with that cold or flu of it getting past your immune system barriers and infecting you is going to be strongly dictated by the different aspects of your immune system”