Craniopagus parasiticus is an extremely rare condition that occurs when an embryo containing identical twins fails to split completely, leaving an underdeveloped parasitic twin that remains attached to a fully developed child.

In February 2005, doctors in Egypt performed the first "successful separation" of a craniopagus parasiticus case. During a 9-month surgery, doctors removed the underdeveloped twin from a 10-month-old child. Throughout the procedure, the child experienced severe bleeding, requiring a staggering four liters of blood. Skin and bone grafts from the parasite were used to repair the wound left by the surgery.

Although child lived for more than a year "without neurological deficit," she died suddenly after coming down with a fever in March 2006.

An infant with craniofacial duplication (also known as diprosopus) has two sets of facial features -- although not necessarily two heads -- and an outwardly regular trunk. The condition's causes are not well understood, although many experts believe that the conditions originates during an "embryological disturbance in the separation of the twins during the 2nd week of pregnancy."