Craniopagus parasiticus is an extremely rare type of parasitic twinning occurring in about 2 to 3 of 5,000,000 births. In craniopagus parasiticus, a parasitic twin head with an undeveloped body is attached to the head of a developed twin. Fewer than a dozen cases of this type of conjoined twin have been documented in literature. Development
The exact development of craniopagus parasiticus is not well known. However, it is known that the underdeveloped twin is a parasitic twin. Parasitic twins are known to occur in utero when monozygotic twins start to develop as an embryo, but the embryo fails to completely split. When this happens, one embryo will dominate development, while the others development is severely altered. The key difference between a parasitic twin and conjoined twins is that in parasitic twins, one twin, the parasite, stops development during gestation, whereas the other twin, the autosite, develops completely.In normal monozygotic twin development, one egg is fertilized by a single sperm. The egg will then completely split into two, normally at the two-cell stage. If the egg splits in the early blastocyst stage, two inner cell masses will be present, eventually leading to the twins sharing the same chorion and placenta, but with separate amnions. However, the egg can split into two, but still have one blastocyst. This will lead to one inner cell mass and one blastocyst. Then, as the twins develop, they will share the same placenta, chorion, and amnion.
A cat named Frankenlouie was a famous example noted by the Guinness Book of World Records in 2012 for being the longest surviving Janus cat. Treatment
Few individuals survive until birth. For those who do, the only treatment available is to surgically remove the parasitic twin. Of the two documented attempts, however, one child died within hours and neither reached their second birthday. The problem with surgical intervention is that the arterial supplies of the head are so intertwined that it is very hard to control the bleeding, but it has been suggested that cutting off the parasitic twins arterial supply might improve the odds of the developed twins survival. Prevalence
Only ten cases of craniopagus parasiticus have been documented, out of a total of at least eighty known cases of craniopagus in general.Only four cases have been documented by modern medicine to have survived birth:

An earlier case was the so-called "Two-Headed Boy of Bengal", who was born in 1783 and died of a cobra bite in 1787. His skull remains in the collection of the Hunterian Museum of the Royal College of Surgeons of London. On December 10, 2003, Rebeca Martínez was born in the Dominican Republic with this rare condition. She was the first baby born with the condition to undergo an operation to remove the second head. She died on February 7, 2004, after an 11-hour operation. On March 30, 2004, Manar Maged was born with the rare condition. On February 19, 2005, 10-month-old Manar underwent a successful 13-hour surgery in Egypt.