Rare diseases seem to be diagnosed by doctors all the time on TV. After all, most of the common stuff is boring, right? But why do TV doctors diagnose some rare maladies and not others? For example, "House M.D." has a common refrain -- "It's never lupus." Why? Systemic lupus erythematosus, usually just referred to as lupus, is a chronic inflammatory disease that causes the body's immune system to damage the person's body. Lupus can cause hair loss, fatigue, memory loss, and fever. These symptoms could account for any number of other diseases. Organs such as the heart and lungs may be affected by lupus. That diverse range of symptoms can cause misdiagnoses. And lupus isn't that common. It occurs in 1.8 to 7.6 Americans per 100,000 [source:U.S. Centers for Disease Control]. And that's why one time -- finally, on an episode entitled "You Don't Want to Know," a patient was finally diagnosed with the rare disease [source: Lupus Foundation of America].