Scientists and physicians experimented with organ transplantation using animals and humans as early as the 18th century. And though replacing a failing organ with a foreign organ has always been a significant surgical feat, it alone couldn’t save lives. Once the new organ was in place, the recipient’s body strenuously objected, mounting a deadly response. For decades, organ rejection was the stumbling block, the thing that drove researchers back to their labs, where they worked for decades looking for answers.

Difficulty walking. Memory problems. Urinary incontinence. Alone or in combination, these symptoms can indicate a variety of diseases and conditions. That’s why making a definitive diagnosis for a collection of symptoms such as these isn’t always easy.

Despite today’s advanced state of medical diagnosis and treatment, there are people with challenging medical mysteries who, even after referrals to multiple specialists and seemingly endless testing, have no answers for the cause of their sometimes-debilitating symptoms.

Most of us have confessed, at one time or another, to feeling depressed. But often what we really mean is: I’m sad. Or maybe, I’m tired. Or bored. Actual depression, also known as major depressive disorder, is more than sad feelings. People living with this disorder may experience, among other symptoms, changes in appetite, suicidal thoughts, difficulty sleeping or oversleeping and a feeling of hopelessness.

For people who experience a stroke, receiving timely medical care can be the difference between recovery and disability. This is especially true for those who experience the most severe type of ischemic stroke, which happens when a blood clot blocks a major artery in the brain.