Diogenes syndrome is a behavioral disorder. It is not a disease, it is a psychological condition. People who have it do not look after themselves enough any more, this is called self-neglect. They do not wash, perhaps do not shave, etc. So they develop a strong odor (scent). Since they neglect their hygiene, other diseases related to poor hygiene usually accompany the disorder.
Very often this disorder affects old people who live alone.
Diogenes Syndrome is named after the behavior (but not necessarily the ideals) of Diogenes of Sinope. It is thought to be the end stage of personality disorder.