The human body is pretty tough most of the time. It overcomes widespread illnesses, such as the flu, and adapts to common chronic conditions, such as diabetes, to function optimally. But there are also diseases and conditions so rare and bizarre that they afflict the body in ways most doctors have only read about in medical literature – diseases and conditions most people don’t even know exist.

To find rare and mysterious medical conditions, 24/7 Tempo reviewed several news and medical sites. A medical condition is defined as rare, according to the Food and Drug Administration, when it affects fewer than 200,000 people in the U.S. All of the diseases on the list are rare by that standard, and some are extremely uncommon. (In contrast, these are the most common ailments in America.)

Click here to see 27 rare and mysterious medical conditions

Among our discoveries: There is a speech disorder that causes people to suddenly start speaking in a foreign accent. Another condition – which may sound like something desirable but that can be very dangerous – prevents people from feeling any physical pain. There is even an illness that makes people believe that they’re dead.

Some of these rare conditions are merely inconvenient or embarrassing, but others can be deadly. (These are the most dangerous diseases you can get on earth.)