The "One Way Flow" of energy that is Reiki gives it a "built in" protection safety mechanism. Here's why: When you connect to Reiki energy with the intention of channeling it, either for yourself or others, the energy flows down from source, enters the crown chakra, goes down through the center channel of the body, past the third eye chakra and throat chakra to the heart chakra where it flows down both arms and into both hands, and out of the minor palm chakras and finger tips.
Reiki never comes back into the practitioner from the "receiver" (the person who we are giving Reiki to) because of this "one way flow" mechanism. The Reiki practitioner never pushes anything into the receiver or is trying in any way to manipulate their energy. They are simply acting as a conduit or vessel for the energy to flow through when they place their hands on the receiver with the intention of channeling Reiki for them during a Reiki session.
However, there are several ways that one can cancel out this protective aspect. If you are fearful that you will take on the disease/s or mental condition of the receiver, your fear can cancel out this protection. Remember, your thoughts create your reality (now even more so than ever with the Shift). So making sure you keep your thoughts absolutely in faith that you are safe and protected when channeling Reiki is paramount.
Another way you can cancel out the built in protection is if you prescribe to the belief system that it is your responsibility to take on the suffering of others. First of all, it's not anyone's job to do that, some refer to this kind of thinking as religious dogma. When performing Reiki, our job is to strictly to act as a conduit for the energy, anything else involves your ego, which is why we don't call ourselves healers. We are only the vessel or conduit for the energy to flow through.
Many massage therapists and other alternative healers become Reiki (take Reiki classes and get attuned to the Reiki energy so that they can channel it for themselves or others) for this reason. It is a well known fact that many massage therapists take on "stuff" from their clients. I hear this all the time from my massage therapy clients, friends, and my own massage therapists. It also adds an extra skill set to their massages when they can claim they are also channeling Reiki energy when they giving someone a massage!
The many benefits of Reiki are vast and some might say miraculous. This is just one of them.