Don't laugh, might be psycho somatic but years ago while I was suffering from a summer head cold a friend told me to start taking 1000mg of Vitamin C every day and double up on it if I felt a cold coming. I did, and in the last 8 years have not had a single cold. Used to have 4 or 5 every year. Nice thing about Vitamin C is you pee out any excess unlike some of the others that can mess you up. At least according to my GP.