One thing that veterinarian medicine has taught me is that there are many reasons why a doggy kiss may be medically unwise. Most of them involve the animal transmitting something from one patient to another, or, just as undesirably, to the therapy dog him/herself. Staph infections or skin fungi such as ringworm can be transmitted in this way, as well as poison ivy or other conditions involving fluid-filled vesicles. Or, a patient may have a contagious illness, either viral or bacterial, that the dog may vector - of critical concern in patients with challenged or damaged immune systems. The patients at greatest risk are those who have undergone transplant procedures.
Medical paraphernalia, such as catheters, hep-locks, bandages, etc. should not be touched by the dog in any way at all, for the dog's sake as well as the patient's. Additionally, some medical procedures and conditions, as well as age, can lead to extremely friable skin. In these cases, even the lightest of licks can cause either pain or damage. Some medications and medical conditions can alter a person's clotting time and sensitivity so radically that even the slightest touch can lead to bruising or spontaneous bleeding. If a patient has recently had even the most minor of surgeries, the healing skin should be avoided. The same holds true for scrapes, scratches and scabs. Studying veterinarian medicine has helped me to see that many patients who would otherwise love to have a dog visit are fearful of physical contact of any kind, for good cause.