Viruses or fungi cause lichen. It is transmitted from a person or an animal by contact. I would not advise to self-medicate, because other people can also get infected. With such a disease, you should contact a dermatovenerologic dispensary, a dermatologist.
It is caused by a fungus. It can usually be contracted from pets. I remember as a child we had a whole epidemic of this deprivation in street cats. I even got infected with a domestic cat (who ran outside) and then infected me (appeared on his hand). After contacting the doctor, I was advised to burn the paper (rolled paper cone) on a ceramic white plate, after which it was necessary to smear lichen with the resulting brown coating. There was no other treatment, but the lichen was gone. But if you become infected, then be sure to consult a doctor. I am typing this and think that the doctor suggested some strange method of treatment. If you are infected, then be sure to consult a doctor, you can ask him about such a method of treatment as I wrote, and use it only with the permission of the doctor. But most likely he will put out some kind of normal ointment, and not a folk remedy.