Having worked in health I've come across some rare diagnoses and I've always been interested in the odd, the macabre and the mystery of our biology. My list of weird disorders that fascinate me (reasons vary):
1. Cotard's Syndrome (The "walking corpse" disorder)
In this, deluded persons think they have lost body parts or their souls, and often believe they have died.
2. Kleinfelter's Syndrome
I like pronouncing the name. Males have an extra X sex chromosome. The principal effect is abnormal testicular/penal development and reduced fertility. A variety of other physical and behavioral differences and problems are common, including a feminine appearance and gynocomastia (man boobs).
3. PKU (Phenylketonuria)
A genetic disorder that is characterized by an inability of the body to utilize the essential amino acid, phenylalanine. Characteristics include failure to attain early developmental milestones, microcephaly (small head), progressive impairment of cerebral function, hyperactivity, EEG abnormalities and seizures, severe mental retardation. A "musty" odour of skin, hair, sweat and urine (due to phenylacetate accumulation); and a tendency to hypopigmentation (loos of skin colour) and eczema are also observed.
4. Body Dysmorphia (extreme cases)
In the extreme case, people believe that certain parts of their body are not their's (more than just self denial the part actually feels foreign). People with this disorder may go to bizarre lengths to remove the part including self amputation.
5. Alice-in-Wonderland Syndrome
A persons sense of time, space and body image are distorted. People may appear tiny or patients may feel that part of their body shape or size has been altered.
6. Factitious Disorder (Munchausen Syndrome)
People deliberately exaggerate and/or fabricate and/or induce physical and/or psychological-behavioral-mental health problems in themselves. The primary purpose of this behavior is to gain some form of internal gratification, such as attention, for themselves.
7. Munchausen By Proxy
Following on, this is where caretakers (mothers and others) deliberately exaggerate and/or fabricate and/or induce physical and/or psychological-behavioral-mental health problems in others (again for their own needs to seek attention)
8. Hydrocephalus (Water on the Brain)
Marked by a build up of CSF in the brain, it is characterized by a large head. I like the word as well as the word 'shunt' (which are used to relieve fluid inside the head).
The strange mixing of senses such that spoken words might taste like food or letters have colours.
There's so many more but this'll do for now...