A cold is not an infection, but an attempt by the body to get rid of internal stressors. These could be anything from electromagnetic energy fields, to indoor or outdoor pollutants, or just sluggishness in getting rid of metabolic wastes. A lack of vitamins A and D are thought to make you more sensitive to these stressors and to get worse symptoms, which may perhaps develop into flu.
The earlier you start to treat a cold, the better. But it sometimes takes a little practice to be able to sense when a cold is starting. The good news is that even if the cold has become well established, natural therapies can do much to alleviate the symptoms and speed up recovery. I take cod liver oil daily now, for its natural vitamin A and D content, so I can't remember the last time I had a cold, but here is what I recommend if a cold starts.