I think I can sort of answer this question! Major caveat here -- I'm a doctor with a healthy interest/obsession in paleopathology, and have absolutely zero useful knowledge of the Torah on ancient Hebrew culture. So after ingesting those grains of salt, I believe you're talking about the Hebrew word tzaraat, which in my New International Version is translated as "defiling skin disease," also associated with "defiling molds" that can effect clothes and your house (Leviticus 13). The King James version translates this as leprosy (https://www.biblegateway.com/passage/?search=Leviticus+13&version=KJV).
There is actually a description of the disease in the book and various Rabinical commentaries -- tzaraat can present as:
Each of these presentations has a more detailed description; for example, lesions on normal skin requiring 1 of 3 secondary changes:
These descriptions are SO tantalizing for doctors to identify the causative disease since they're relatively specific. Traditionally, tzaraat was thought to be leprosy (hence the KJV translation). I've written about "historical leprosy" before on r/AskHistorians. Essentially the TL;DR is that many historical descriptions of leprosy almost certainly did NOT have Hansen's disease (caused by M. leprae). Tzaraat is no different; patient's with leprosy can have hypopigmented skin lesions on previously normal skin, as well as hair loss. But it's pretty much impossible to fit all four descriptors into Hansen's disease (and this description would lose some of the most salient features, like the nerve findings).
If it's not leprosy, what is it? Any number of skin conditions fit the bill, and doctors have submitted almost all of them -- pellagra (Niacin deficiency), favus (horrific fungal skin infection that was very common in the pre-modern world), other fungal skin infections (seborrheic dermatitis and its fellows), syphilis (almost impossible since it was confined to the Americas at that time), and then of course vitiligo. Vitiligo (https://en.wikipedia.org/wiki/Vitiligo#/media/File:Vitiligo2.JPG) is an autoimmune depigmenting skin disease that could potentially fit the description.
Some problems -- and why I think that most of the attempts at paleopathology here are wrong-headed (most seem to be done by Torah scholars, some of whom also have medical training). The Leviticus chapters of tzaraat come from a premodern people -- but also a pre-medical people (a big aside, while this is a gross simplification, the Hippocratics are usually associated with the idea that disease is natural, and not supernatural. That being said, we have textual evidence that other Mediterranean people, especially the Egyptians ~1500 BCE had a dichotomous view -- some disease is natural, but other has spiritual causes). That is, reading through Leviticus, there does not seem to be a sense that the writers felt that tzaraat had natural causes. It is primarily seen as a spiritual disease, to be treated with ritual cleansing in similar ways to other spiritual diseases. And it's spiritual taint is not limited to the human body -- tzaraat ALSO affects the walls of houses, and clothes! If you include those diagnostic factors in our discussion, there is absolutely nothing that fits this description.
So here's my personal paleopathological take away (which seems to be the modern consensus) -- tzaraat does not refer to any skin condition that we know of today, and is a ancient people's attempt to make sense of what was likely a hodgepodge of skin conditions, an ancient people with a fundamentally different relationship with the natural world and disease than we have. Was it common? Presumably, since it occupies a significant part of Leviticus. Skin conditions occupy a significant portion of the Hippocratic corpus (which, while it has a naturalistic nosology, is still fundamentally strange to modern medicine). What I'll say is that fungal diseases in particular are still common today, and prior to modern sanitation were widespread -- favus in particular (https://www.google.com/search?q=favus&rlz=1C1GKLA_enUS756US756&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjJ4K2uovzeAhUshuAKHTaxCwQQ_AUIDigB&biw=547&bih=548; that's a google image search of the condition, consider yourself warned). I used to work in Botswana as a doctor in a rural area, and have personally seen how dramatically what are thought to be benign fungal infections can present.
Okay, I hope that was helpful. If you want more, this is an academic article (https://www.ncbi.nlm.nih.gov/pubmed/7045170) by a dermatologist that goes into this in more detail.
