[krynski]

It really interests me that people like to argue about this all the time. Colds happen because the immune system is compromised in some way or another, making virus proliferation more possible than under regular circumstances. Cold temperatures decrease basal metabolic rate and simply make it more difficult to do some cellular functions, making cells more prone to viruses, (im not exactly sure why this is, maybe this is because it delays programmed cell death response?). Fatigue will also have a similar effect, along with long-term stress.

EDIT: a strong immune system does not erradicate all of each virus within seconds of system entry, it just makes the response quicker and stronger.