Pain vampires or dureri nosferatu -meaning the pained disease carriers- are the most commonly known, and so far the most elusive types of vampires. The first record of humans becoming addicted to the pain of others is unknown. Texts make references to sadists as far back as the origin of the human psyche.
Physically, they are human. They consume food, have warm blood and a fully functioning bodily system. They are not 'undead' as one may assume, but rather diseased, though it is doubtful that they view themselves as such.
They receive life from the pain of others, or rather, they postpone death with the pain of others.
The Process Edit
There is a chemical compound that modern medicine calls 'substance P'. This is an amino acid that is released when one is in severe pain. It is a neurotransmitter and neuromodulator. Of course, substance P also has many positive affects on the injured area: increased blood flow, increased cellular regeneration, anti inflammation, increased insulin production, vomit regulation, and even as a mood enhancer by increasing stimulation to the neuropathways.
In normal humans, substance P is seen as a cause in many cased of hypersensativity and even eczema. However, in the case of these creatures, they have been overdosed - often times over the course of several weeks - with substance P, to the point where their physiology evolves and needs high levels to function. Of course, in this changing process, their body begins to produce natural opioids to compensate for what the brain would interpret as an alarming state of pain.
These opioids are transmittable through the skin, and in extreme cases, through general proximity. Normal humans react to this very much in the way you might expect.