Histiocytosis is a rare blood disease that is caused by an excess of white blood cells called histiocytes. The histiocytes cluster together and can attack the skin, bones, lung, liver, spleen, gums, ears, eyes, and/or the central nervous system. The disease can range from limited involvement that spontaneously regresses to progressive multiorgan involvement that can be chronic and debilitating. In some cases, the disease can be life-threatening.

In some ways, histiocytosis is similar to cancer and has historically been treated by oncologists with chemotherapy and radiation. Unlike cancer, histiocytosis sometimes goes into remission without treatment.

The vast majority of people diagnosed with histiocytosis are children under the age of 10, but it is also found in adults of all ages.

It is approximated that histiocytosis affects 1 in 200,000 children born each year in the United States. This illness is so rare, there is little research into its cause and treatment, and it is often referred to as an "orphan disease," meaning it strikes too few people to generate government - supported research.