Garlic and zinc I've heard help prevent viral replication. Zinc especially is s'posed to help the first 2 or 3 days of a cold. (The sulpher compounds in garlic are supposedly antibiotic, too. Garlic for everyone!) So you'll catch the little boogers but the garlic and zinc will help to slow them down and your immune system does the rest.
My first sign of a cold is tonsil grumpiness. That's when I start on the zinc. A couple days later the actual cold begins and I quit the zinc. I figure by then it's up to my immune system and I do what I can to support the immune system and treat my symptoms.
And I still want to believe that there are a limited number of cold viruses out there, and after you've had them all you won't catch anymore. Allow me to live in my happy fantasy world...
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson