Diagnosing diseases is a tricky business requiring a formidable breadth and depth of knowledge and the skill to apply that knowledge. Patient diagnosis becomes even more difficult for rare diseases: quality reference data may not exist and a physician might only see one such patient in her entire career. According to the National Institutes of Health, there are between 6,000 and 7,000 rare diseases affecting from 25 to 30 million Americans , making it likely that most, if not all, health care professionals have seen these patients in their practice but may not have known it. Oftentimes, a patient with a rare disease gets misdiagnosed as having a more common disease with a similar set of symptoms. In such cases, the misdiagnosis can lead to ineffective, or even harmful treatment; this is a danger even for patients who have rarer forms of a common disease. Next Tuesday is Rare Disease Day , a day devoted to raising awareness of rare diseases, learning from the patients and families livin...