Colds are caused by a rhino-virus, There are many different kinds of these viruses and few people have the anti-bodies to protect against all of them. When the immune system is compromised then fewer antibodies are produced. Vitamin C helps strengthen the immune system. I have wondered if perhaps the large doses of Vit. C taken after the onset of a common cold might actually create an acidic environment that will not support the cold virus. Remember that the acid mantle of the skin is said to protect against bacterial infection; and the bladder and colon both need to be on the acidic side to protect from harmful organisms. The acidophilus bacteria needed in a healthy colon is "Acid loving" in Latin. This subject is far more complicated than a reading on a pH strip, and different systems and organs have different requirements for their acid levels.