Situs inversus (also called situs transversus or oppositus) is a congenital condition in which the major visceral organs are reversed or mirrored from their normal positions. The normal arrangement of internal organs is known as situs solitus. Although cardiac problems are more common, many people with situs inversus have no medical symptoms or complications resulting from the condition, and until the advent of modern medicine, it was usually undiagnosed.

Hyperdontia is a condition that’s characterized as having supernumerary teeth, or an excess number of teeth. Although the cause is unknown for sure, there is believed to be a genetic factor in hyperdontia as well as overactivity of the dental lamina during tooth development.

Persistent Sexual Arousal Syndrome is a condition, which is mostly found in females, is defined as having uncontrollable genital arousal. It’s not linked to sexual desire or sexual stimulation, and a woman can often have multiple orgasms that do not relieve the tension.

Cold urticaria is a cold allergy where people develop itchy hives when exposed to low temperatures. The condition is most commonly found in young adults, and symptoms can range from mild to quite severe.

Hematohidrosis is a very rare condition whose sufferers sweat blood. A 21-year-old Italian woman with the disorder said that the bleeding would become more intense when she was stressed, and it could last up to five minutes.

Fibrodysplasia ossificans progressiva is an extremely rare condition that affects about 1 in 2 million people. Simply stated, the condition turns muscle into bone, and there is currently no cure.

Dysegeusia is a condition in which a person will experience a metallic or foul taste in their mouth. Sometimes, it can come with “burning mouth syndrome” which is when a person feels as if they’ve just burned their mouth.

People with the neurological condition called Sleeping Beauty syndrome experience periods of excessive sleep. These episodes can last up to 20 hours a day, and bouts may go on for days, even weeks, according to the National Institute of Neurological Disorders and Stroke (NINDS). These episodes of lengthy sleep may be proceeded by flu-like symptoms and, when they're awake, people with this syndrome may exhibit various odd behaviors, including eating excessively, hallucinating or acting childishly.

Flesh-eating bacteria is exactly what it sounds like, a bacterial skin infection that eats away at soft tissue called the fascia, which surround muscles, blood vessels, nerves and fat. It also damages tissue near the fascia. Many different bacteria can cause this infection, but Group A strep (Streptococcus) are the most common cause. The infection can spread quickly and be deadly but, if caught early, it can be treated with antibiotics and surgery.

Necrotizing fasciitis is rare, especially among healthy people, according to the Centers for Disease Control and Prevention. People looking to avoid it should practice proper wound care and stay away from hot tubs, swimming pools and natural bodies of water (like lakes, rivers and oceans) if they have an open wound or infection.

Congenital insensitivity to pain: Pain is a life-saving physical response to danger, and when people are born with a rare genetic mutation that leaves them unable to feel pain, they often die early deaths as a result of treatable injuries that they simply fail to notice. It all starts in infancy, when babies born with pain insensitivity bite off the tips of their tongues, break their bones without making a fuss, and get corneal damage after neglecting to brush foreign objects out of their eyes.