It gets biased because a single positive feeling is enough to get the human mind to think there is a causal connection between the pill and the "cure". There a

It gets biased because a single positive feeling is enough to get the human mind to think there is a causal connection between the pill and the "cure". There are hundreds of variants of the common cold virus, and you are at any time more or less immune to a subset of them, so many times after you're exposed you just feel ill for half a day. If you pair one of those occurances with your magic pill intake, you'll truly believe the pill caused you to get better, and then you'll keep buying that pill every time you feel a cold coming up for the rest of your life. A real medical study is 10,000-100,000 patients, it's not possible to draw any conclusions from a single human's observation in this regard... but people keep trying, it's due to the nature of the human mind and its analysis of causality :)