They are conditions that one would be hard-pressed to find in medical literature.
Bring them up in front of a physician and in some cases you may get little more than a blank stare.
But they exist, often as an extreme form of a normal bodily function that most people experience every day.
And while the thought of uncontrollable hiccups or never feeling pain sensations are cocktail-party fodder for some, they can be a source of difficulty and shame for those who experience them firsthand.
People with these strange conditions can be so uncomfortable, physically, socially and emotionally, that their lives can grind to a halt as they struggle to find a cure.
The following pages feature some of the more unusual medical conditions that have received recent media attention.
Christopher Sands, 25, has been battling hiccups, normally a mundane, short-lived biological function, for more than two years.
"When [the hiccups] started, it was completely random, out of the blue, for no reason," said Sands, whose first bout of chronic hiccupping occurred in February 2007.
"In general, [hiccupping] is a sign of diaphragm-related problems," said Dr. Martin Makary, director of the Johns Hopkins Center for Surgical Outcomes Research, who pointed out that the muscle can contract irregularly due to irritation or an abnormality such as a hole.
But it quickly became clear that nothing worked to alleviate Sands' hiccups. After much medical testing, including head, chest and abdomen scans, doctors were unable to identify the source of Sands' chronic hiccups.
The only thing wrong was a damaged valve connecting Sands' esophagus to his stomach, a condition that he was born with and that caused him frequent heartburn and vomiting. Coupled with the hiccups, Sands has not been able to eat, sleep or perform as a guitarist and backup vocalist with his band, Ebullient.
"I can hiccup for 14 hours nonstop or I'll be free of them, but not free of them at all," Sands said.
When he is not actively hiccupping, Sands feels like he is about to hiccup at any moment, much like the tickle before a sneeze.
"I'm curled up in a ball on the floor, writhing around in pain, drinking water."
Sands had surgery this week to correct the faulty valve, but it had no effect on his condition.
BBC1 is working to develop a program featuring Sands as he travels across the United States seeking a cure for his hiccups.
Natalie Adler, 21, of Caulfield South, Melbourne, Australia, said she can sense the onset of her unusual condition -- and when she should start preparing for three days of darkness.
"The night before it sets in, my eyes get quite heavy and that is how I know it is coming," Adler told ABCNews.com.
Invariably, the next morning Adler finds herself unable to open her tightly shut eyes. The bouts generally last for about three days, after which she can open her eyes and once again see normally.
Adler has suffered from the condition for the last four years, she said. Doctors, both in Australia and the United States, are baffled as to the exact cause -- or indeed, whether her condition is physical or psychological.