Congenital hypertrichosis lanuginosa (HLC) is a rare condition in which lanugo (fine hair that covers the body of the fetus and disappears shortly after birth) is not replaced by normal hair, it even continues to grow until reaching 10 cm throughout body surface except palms, soles, dorsal surface of distal phalanges and foreskin.
The life expectancy of these people is usually the same as that of a normal person, but society makes the person suffering from this disease to be isolated and suffer discrimination and physical and psychological abuse.
That of Petrus Gonsalvus was the oldest case of Hypertrichosis of those described in Europe.
Another explanation for werewolf syndrome is the Late Cutaneous Porphyria, a disease where Porphyrins (intermediates in the synthesis of Heme, accumulate since enzymes are missing to complete their synthesis) in the skin, which in contact with the UV radiation of the sun they generate excitation in the molecules, releasing free radicals (harmful to the tissues) and these generate damage to the skin. A defense mechanism of the body is to increase the amount of hair in the areas of contact with this type of radiation to avoid damage.
People with this disease (not diagnosed or known) went out at night to avoid the sun, with more hair than usual and on full moon nights to be able to see under its light (there was no electric power or flashlights)
In this way, many myths about werewolf would have a possible explanation.
* Excerpted from the book “Trichology (chap 6, vol 1) by Dr. Montagna and Dr. Tosti.