It is obvious that we are born to be healthy. The human body is designed in such a way as to be healthy even in harsh environments. There are plenty of examples which can sustain this statement. For example, there are always bacteria and viruses inside the human body, yet most of them are neutralized and they do not get to spread and cause problems. Moreover, they get to be used and exploited for our own being. Also, toxins reach our body, but they can be quickly eliminated, just as the brain continues to function normally in stress conditions. Yet, there are times when these situations get out of control and the body can no longer cope with their invasion. Why?
There is a simple answer for that: when functioning normally, the body is trespassed by a flow of vital energy which prompts each organ to work as it should. When imbalances occur and this flow is disrupted, the vital energy can no longer reach all organs, thus leaving them without the necessary support for functioning as they should. This results in illnesses and diseases at physical or intellectual level. What Reiki does is to reestablish that flow of positive energy, so that each and every organ will receive as much energy as it needs to recover and retake its normal functioning.
For this, the practitioner will need to channel the energy towards your body and then guide it to the source of your problems. If there are unspotted places where the practitioner may not guide the energy, then the flow of energy will immediately direct towards those spots, so there is no need to worry about whether the practitioner will find the right spot or not. The vital energy is guided by the supernatural force which created universe, so it will feel what organs need more positive energy. Also, the body will only acquire the amount of energy it needs to function properly, so the practitioner should not refrain from letting the energy flow towards the client.
Reiki does not use any type of substances, is non-invasive and does not affect the body in any negative way. The energy it uses is positive and comes from the creator of this world, so it cannot be used for negative purposes. Because of that it can be used both for healing and for prevention.