I believe your question is about the heat liberated fro tissues.
Human body liberates heat normally as a part of body metabolism. But science has discovered that if our body have some infections , cancer cells or any other deformities especially in tissue growth, that part of body will have different concentration of blood flow and hence thereby that part of body emit different temperature.
This variations of heat liberated by human bod can be measured and can be used for diagnosis by Medical Thermography technique. You can browse about medical thermography to get more details about it.