Think of a sick person as one of Pavlov's dogs. Some experts say that a sugar pill is the proverbial bell -- you take a pill, like ibuprofen for a muscle ache, the active ingredient enters your bloodstream, and you get better. This has happened many times. So when you take a pill that contains no active ingredients, but you think it does, you still get better! You thought you were so smart, did you? Your body just relieved your symptoms for no reason whatsoever, except that it knows what a pill is supposed to do.
This is your brain on (fake) drugs
The placebo effect has been a known phenomenon for a long time, and it's opened the door to a string of medical, philosophical, and anthropological opinions that are difficult to prove definitively one way or another. Like in any healthy scientific debate, people who believe one thing or the other are hitting each other over the head with academic journals about this, but there's a pretty good argument that either/both of the above theories can be true, depending on the situation.