Seán Manchester (from his writings) on werewolfism:
The Vampire Research Society has an interest in werewolfism and has investigated suspected cases in the British Isles and France, but it is impossible to know how common the affliction might be in such places as America and parts of Europe generally. This is largely because the lycanthrope/werewolf can fall under a number of different categories, some of them medical, and is of a different kidney to the vampire. I met a woman in Highgate in the 1980s who believed she was turning into a werewolf, but actually suffered from an extreme form of lupus syndrome which is a chronic inflammatory disease that occurs when the body's immune system attacks its own tissues and organs (auto-immunity). Inflammation caused by lupus syndrome can affect many different body systems, including joints, skin, kidneys, blood cells, heart, and lungs. It occurs more frequently in women than in men, although the reasons for this are unknown. Four types of the condition exist — systemic lupus erythematosus, discoid lupus erythematosus, drug-induced lupus erythematosus and neo-natal lupus syndrome. Of these, systemic lupus erythematosus is the most common and serious form of lupus syndrome. These are medical conditions and not werewolfism.
Some people with lupus syndrome also have problems with their blood clotting too quickly. These people have anti-phospholipid antibody syndrome, lupus anti-coagulant or anti-cardiolipin. The condition is managed with blood thinner like coumadin or wafarin and must be carefully monitored. Lupus erythematosus is a connective tissue disease. There is also a mental illness called lycanthropy in which a patient believes he or she is, or has transformed into, an animal and behaves accordingly. This is sometimes referred to as clinical lycanthropy to distinguish it from its folkloric counterpart where the person has the apparent ability or power of a human being to undergo transformation into a wolf, or to gain wolf-like characteristics.
The term lycanthropy comes from the Greek lykánthropos (λυκάνθρωπος): λύκος, lýkos ("wolf") plus άνθρωπος, ánthrōpos ("human"). It is sometimes used generically for any transformation of a human into animal form, though the precise term for that is technically therianthropy. The werewolf may be regarded as a man or woman who, either of his or her own will through the black arts, is able to assume the hideous appetite, ferocity, cunning, and other qualities of the wolf; so that he or she will attack human beings in the same way as a wild animal. There are recorded instances where the person has taken on a wolf-like appearance. Werewolfism can be hereditary, or acquired through a demonic agency, but, unlike the vampire, werewolves are living persons either afflicted, or self-afflicted, with the malady that sometimes results in an apparent transformation. Vampires, on the other hand, are demonic entities in apparent corporeal form which manifest at night to feed of the blood of the living whereby their material appearance is maintained and indeed nourished. Werewolves, on the other hand, are people who assume a wolf-form and wolf-like behaviour.
The lycanthrope werewolf should not be confused with the voluntary werewolf, under whom for this consideration any form of apparent shape-shifting may be included. An essential prerequisite is a pact, formal or tacit, with a demonic agency. Such metamorphosis as that examplified in the voluntary werewolf can only be wrought by engagement in the dark arts. Shapeshifting is certainly not uncommon where demonic agencies are involved, and I have encountered this phenomenon in the course of my research and investigations.