Only ten cases of craniopagus parasiticus have been reported in the medical research literature. Of those cases, only three have survived birth. The first case on record is that of Everard Home's Two-Headed Boy of Bengal, whose skull is preserved at the Hunterian Museum at the Royal Society of Surgeons.

In the past, the use of terminology when describing parasitic twins has been somewhat inconsistent. By definition, a parasitic twin is joined to another twin in a certain anatomical location or position on the developed twin's body. The underdeveloped twin is termed the parasite, and the developed twin is termed the autosite. The autosite can have some abnormalities, as well. For the most part, however, it has developed enough that it can live on its own.

Craniopagus twins are conjoined twins that are fused at the cranium. This condition occurs in about 10–20 babies in every million births in the United States. Among this small group, cephalic conjoining, or craniopagus twinning, represents the rarest of congenital abnormalities, accounting for 2–6% of all conjoined twins. Additionally, conjoined twins are genetically identical and always share the same sex. The union in craniopagus twins may occur on any portion of the Calvary, but does not include either the face or the foramen magnum. The thorax and abdomen are separate and each twin has its own umbilicus and umbilical cord. The union may involve the entire diameter of the head or only a small portion. This suggests that although there are many different kinds of vulnerabilities already known in the scientific community, there are an infinite number of variations that can occur. Most of these variations are based on the rotation of one twin's skull to the other and the different phenotype sub-groups of craniopagus twins are based on all these rotational conformations. Each of these factors (rotation, spot of union) affects the development of the brain, the vascular system within the brain and overall wellness of life both of the twins have outside the womb. Relatively few craniopagus twins survive the perinatal period – approximately 40% of conjoined twins are stillborn and an additional 33% die within the immediate perinatal period, usually from organ abnormalities and failure. However 25% of craniopagus twins survive and can be considered for a surgical separation and several attempts occur yearly worldwide. In the last-half century, many advances in medicine including brain imaging, neuro-anesthesia and neurosurgical techniques have proven that a successful outcome is possible following separation of total craniopagus twins.

Embryo splitting in which zygote divide asexually,

to produce identical children, is blocked by mitosis inhibitor.

"'Conjoined twins" are identical twins joined in utero. An extremely rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence in Southeast Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.

Two contradicting theories exist to explain the origins of conjoined twins. The more generally accepted theory is "fission", in which the fertilized egg splits partially. The other theory, no longer believed to be the basis of conjoined twinning, is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to conjoined twins as there are some monozygotic but non-conjoined twins who also share these structures in utero.

The most famous pair of conjoined twins was Chang and Eng Bunker (Thai: อิน-จัน, In-Chan) (1811–1874), Thai brothers born in Siam, now Thailand. They traveled with P.T. Barnum's circus for many years and were labeled as the Siamese twins. Chang and Eng were joined at the torso by a band of flesh, cartilage, and their fused livers. In modern times, they could have been easily separated. Due to the brothers' fame and the rarity of the condition, the term "Siamese twins" came to be used as a synonym for conjoined twins.

Two-headed people and animals, though rare, have long been known to exist and documented.

In humans, as in other animals, partial twinning can result in formation of two heads supported by a single torso. Two ways this can happen are dicephalus parapagus, where there are two heads side by side, and craniopagus parasiticus, where the heads are joined directly.

The exact nature of how conjoined twins develop inutero remains unclear. Embryologists have traditionally attributed identical twinning as "splitting or fission" of either the inner cell mass of pleuripotential cells or early embryonic disc at 13–14 days of gestation just before the primitive streak. Some theorists suggested that conjoined twins develop as a result of the failed fusion of a single fertilized ovum. However a new hypothesis suggests that cranial fusion occurs between two separate embryos prior to the end of the 4th week of gestation. This is able to happen because the cranial neuropore is still open which is responsible for the ultimate fusion and formation of the brain stem and central nervous system. Furthermore, this secondary fusion of embryonic discs could implicate that intact skin will not fuse to other intact skin, including the ectoderm of the embryo. This means that two embryonic discs could only unite in locations where the ectoderm is absent. Moreover, the fusion occurs from neural folds of two separate, dorsally oriented embryonic discs, and the union can occur only after the ectoderm is disrupted to allow the neural and surface ectodermal layers to separate from each other.

The union in craniopagus twins may happen at any portion of the calvarium. The juncture can involve either the entire diameter of the head or any portion of the head and can be positioned at a multitude of rotational angles. In fact, craniopagus twins are rarely found in a symmetrical union. Apart from this, the vertebral axes may have a straight line. Despite this, the angle of the vertebrae is the ultimate dictator in how the individuals heads actually face. The majority of twins face either the same way or the exact opposite direction.

Many reviews suggest a practical four-category system that breaks down the craniopagus twins on the basis of vertical or angular configuration or on the basis if there were shared dural venous sinuses. This scheme was applied to 64 cases and has adequately described sets of twins for over the last 86 years.

Krista and Tatiana Hogan (born October 25, 2006) are Canadians who are conjoined craniopagus twins. They are joined at the head (the top, back, and sides). They were born in Vancouver, British Columbia and are the only unseparated ones of that type currently alive in Canada. They live with their mother, Felicia Simms, in Vernon, British Columbia and often travel to Vancouver for care at BC Children's Hospital and Sunny Hill Health Centre for Children.

Anastasia and Tatiana Dogaru

born August 29, 2004) are craniopagus conjoined twins. They were scheduled to begin the first of several surgeries to separate them at Rainbow Babies and Children's Medical Center in Cleveland, Ohio. However, in August 2007 the surgery was called off as too dangerous.

The twins were born in Rome, Italy to Romanian parents, Alin Dogaru, a Byzantine Catholic priest, and Claudia Dogaru, a nurse. Their mother heard about the successful separation of Egyptian-born twins who were also joined at the head and hoped her children could also be successfully separated. The Dogaru family — who also have an older daughter, Maria, and younger son Theodor — were brought to north Texas by the World Craniofacial Foundation to have Anastasia and Tatiana evaluated for possible separation.

The girls are currently developing normally for their age and speak both Romanian and English. They get around with Anastasia leading the way and Tatiana following. The top of Tatiana's head is attached to the back of Anastasia's. Anastasia, whose kidneys don't function, relies on her sister's kidneys, and Tatiana on her sister's circulatory system. The girls also share blood flow to the back of the brain and some brain matter. Doctors estimated the twins had only a 50 percent chance of surviving the surgery. There were also risks of complications, such as brain damage, but the girls also risk early death if they remain conjoined. Their parents believed separation would give them their best chance at living a normal life.

In May 2007, doctors used a catheter to insert wire coils into the veins of the two girls, successfully redirecting their blood flow. It was the first time the procedure was attempted in conjoined twins. Doctors pushed back the first of the planned separation surgeries to June 2007 while studying the complex circulatory system of the twins, but, in August of that year, decided it was too risky.

Clarence and Carl Aguirre (born April 21, 2002) are former conjoined twins born in Silay City, Philippines. They were conjoined at the top of the head (vertical craniopagus). In 2003 they were brought to the United States by their mother Arlene in the hope of having them surgically separated. A revolutionary new process was used in separating the twin boys. Because "marathon" surgical operations have historically led to a high rate of mortality and morbidity, Clarence and Carl's doctors chose to separate them in several smaller operations, allowing the twins to recuperate after each surgery.

In October 2003, tissue expanders (pouches filled with saline) were implanted under the boys' scalps. This is common practice in separating conjoined twins, as it creates more skin that can be used to cover the twins' wounds after separation. Over the next ten months, the brothers underwent several more operations to tease apart their joined skulls and brains. On August 4, 2004, the final surgery was completed. All of these operations were performed at Montefiore Medical Center in the Bronx, New York by a team led by Dr. James T. Goodrich, a pediatric neurosurgeon and Dr. David A. Staffenberg, a plastic surgeon. They received most of their post-operative rehabilitation at Blythedale Children's Hospital in Valhalla, New York (Westchester County).

By December 2005, Clarence could walk short distances unassisted, and Carl could walk with a walker. As of August 2014 both boys and their mother live in a donated house in Scarsdale, New York. The twins continue to attend physical therapy at Blythedale. Goodrich says there was some degeneration of Carl’s right parietal lobe: he takes medication to control seizures, can utter just a word or two at a time, and has limited use of his left arm and leg. Both must still wear helmets to protect their brains; when they are fully grown, their skulls will be patched.

Based on statistics, the twins were given a 20% chance of survival at birth . At birth at B.C. Women's Hospital & Health Centre, they were described as "wriggly, vigorous and very vocal." They weighed twelve and a half pounds, not six and a half pounds as reported by some media outlets, when they were born by caesarean section.

Lori and George Schappell (born as Lori and Dori Schappell, September 18, 1961, in Sinking Spring, Pennsylvania) are conjoined twins. George has performed as a country singer. In 2007, George, who was at that time known as Reba Schappell, stated that although born female, he identifies as male and changed his name to George.

Globally, infants are a population that are especially vulnerable to foodborne disease. The World Health Organization has issued recommendations for the preparation, use and storage of prepared formulas. Breastfeeding remains the best preventative measure for protection of foodborne infections in infants.

The term alimentary mycotoxicoses refers to the effect of poisoning by Mycotoxins (The term 'mycotoxin' is usually reserved for the toxic chemical products produced by fungi that readily colonize crops) through food consumption. Mycotoxins sometimes have important effects on human and animal health. For example, an outbreak which occurred in the UK in 1960 caused the death of 100,000 turkeys which had consumed aflatoxin-contaminated peanut meal. In the USSR in World War II, 5,000 people died due to Alimentary Toxic Aleukia (ALA). The common foodborne Mycotoxins include:

- Aflatoxins – originated from Aspergillus parasiticus and Aspergillus flavus. They are frequently found in tree nuts, peanuts, maize, sorghum and other oilseeds, including corn and cottonseeds. The pronounced forms of Aflatoxins are those of B1, B2, G1, and G2, amongst which Aflatoxin B1 predominantly targets the liver, which will result in necrosis, cirrhosis, and carcinoma. In the US, the acceptable level of total aflatoxins in foods is less than 20 μg/kg, except for Aflatoxin M1 in milk, which should be less than 0.5 μg/kg. The official document can be found at FDA's website.

- Altertoxins – are those of Alternariol (AOH), Alternariol methyl ether (AME), Altenuene (ALT), Altertoxin-1 (ATX-1), Tenuazonic acid (TeA) and Radicinin (RAD), originated from Alternaria spp. Some of the toxins can be present in sorghum, ragi, wheat and tomatoes. Some research has shown that the toxins can be easily cross-contaminated between grain commodities, suggesting that manufacturing and storage of grain commodities is a critical practice.

- Citrinin

- Citreoviridin

- Cyclopiazonic acid

- Cytochalasins

- Ergot alkaloids / Ergopeptine alkaloids – Ergotamine

- Fumonisins – Crop corn can be easily contaminated by the fungi Fusarium moniliforme, and its Fumonisin B1 will cause Leukoencephalomalacia (LEM) in horses, Pulmonary edema syndrome (PES) in pigs, liver cancer in rats and Esophageal cancer in humans. For human and animal health, both the FDA and the EC have regulated the content levels of toxins in food and animal feed.

- Fusaric acid

- Fusarochromanone

- Kojic acid

- Lolitrem alkaloids

- Moniliformin

- 3-Nitropropionic acid

- Nivalenol

- Ochratoxins – In Australia, The Limit of Reporting (LOR) level for Ochratoxin A (OTA) analyses in 20th Australian Total Diet Survey was 1 µg/kg, whereas the EC restricts the content of OTA to 5 µg/kg in cereal commodities, 3 µg/kg in processed products and 10 µg/kg in dried vine fruits.

- Oosporeine

- Patulin – Currently, this toxin has been advisably regulated on fruit products. The EC and the FDA have limited it to under 50 µg/kg for fruit juice and fruit nectar, while limits of 25 µg/kg for solid-contained fruit products and 10 µg/kg for baby foods were specified by the EC.

- Phomopsins

- Sporidesmin A

- Sterigmatocystin

- Tremorgenic mycotoxins – Five of them have been reported to be associated with molds found in fermented meats. These are Fumitremorgen B, Paxilline, Penitrem A, Verrucosidin, and Verruculogen.

- Trichothecenes – sourced from Cephalosporium, Fusarium, Myrothecium, Stachybotrys and Trichoderma. The toxins are usually found in molded maize, wheat, corn, peanuts and rice, or animal feed of hay and straw. Four trichothecenes, T-2 toxin, HT-2 toxin, diacetoxyscirpenol (DAS) and deoxynivalenol (DON) have been most commonly encountered by humans and animals. The consequences of oral intake of, or dermal exposure to, the toxins will result in Alimentary toxic aleukia, neutropenia, aplastic anemia, thrombocytopenia and/or skin irritation. In 1993, the FDA issued a document for the content limits of DON in food and animal feed at an advisory level. In 2003, US published a patent that is very promising for farmers to produce a trichothecene-resistant crop.

- Zearalenone

- Zearalenols

Aflatoxin exposure can lead to the development of HCC. The aflatoxins are a group of chemicals produced by the fungi "Aspergillus flavus" (the name comes from "A. flavus" toxin) and "A. parasiticus". Food contamination by the fungi leads to ingestion of the chemicals, which are very toxic to the liver. Common foodstuffs contaminated with the toxins are cereals, peanuts and other vegetables. Contamination of food is common in Africa, South-East Asia and China. Concurrent HBV infection and aflatoxin exposure increases the risk of liver cancer to over three times that seen in HBV infected individuals without aflatoxin exposure. The mechanism by which aflatoxins cause cancer is through genetic mutation of a gene required for the prevention of cancer: p53.

In addition to virus-related cirrhosis described above, other causes of cirrhosis can lead to HCC. Alcohol intake correlates with risk of HCC, and the risk is far greater in individuals with an alcohol-induced cirrhotic liver. There are a few disorders that are known to cause cirrhosis and lead to cancer, including hereditary hemochromatosis and primary biliary cirrhosis.

Born as Lori and Dori Schappell, they are craniopagus conjoined twins joined at the head, but having very different personalities and living—insofar as possible—individual lives. As a mark of individuality, and disliking the fact that their names rhymed, Dori first chose to go by the name Reba, after his favorite singer Reba McEntire. By 2007 he preferred to be publicly known as George.

Lori and George spent the first 24 years of their lives living in an institution in Hamburg, Pennsylvania in which the majority of patients were suffering from severe intellectual disabilities. Although neither is intellectually disabled, George's physical condition required special care. A court decision was made that their parents would be unable to care for them properly and they were removed and institutionalized. In the 1960s there were few hospital institutions for people who had special needs that were particularly unusual. In order that they might be placed in the institution, they were diagnosed as suffering from intellectual disability. When they reached adulthood, George, with the help of Ginny Thornburgh, wife of former Pennsylvania Governor Richard Thornburgh, fought to have this diagnosis overturned and they were able to go to college.

While Lori is able-bodied, George has spina bifida, which has caused growth restriction of his lower body and severe mobility impairment. They are therefore of very different heights with her being 5' 1" and him 4'4". There was no wheelchair that suited his unique condition, because to move around, he must be raised to her height, to avoid undue strain upon his neck and back. The only thing on wheels that was the right height was a bar stool. Using this as the foundation, he designed the wheelchair that he currently uses.

Lori and George live in a two-bedroom apartment, each maintaining their own private space. He has several pets. She is a trophy-winning bowler. They respect each other’s privacy in terms of work time, recreation and relationships. She has had several boyfriends and was engaged, but lost her fiance in a motor-vehicle accident.

In 2006, George was baptized a member of the Church of Jesus Christ of Latter-day Saints, in Reading. Lori did not join, but has been supportive of his decision. In 2007, he decided to openly acknowledge that he was transgender, having self-identified as male from a young age.