Craniopagus parasiticus is an extremely rare type of parasitic twinning occurring in about 4 to 6 of 10,000,000 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 the literature. Most infants with this condition are stillborn, or die shortly after birth.

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.

Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:

- Thoraco-omphalopagus (28% of cases): Two bodies fused from the upper chest to the lower chest. These twins usually share a heart, and may also share the liver or part of the digestive system.

- Thoracopagus (18.5%): Two bodies fused from the upper thorax to lower belly. The heart is always involved in these cases. As of 2015, separation of a genuinely shared heart has not offered survival to two twins; a designated twin may survive if allotted the heart, sacrificing the other twin.

- Omphalopagus (10%): Two bodies fused at the lower abdomen. Unlike thoracopagus, the heart is never involved in these cases; however, the twins often share a liver, digestive system, diaphragm and other organs.

- Parasitic twins (10%): Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin for survival.

- Craniopagus (6%): Fused skulls, but separate bodies. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.

Other less-common types of conjoined twins include:

- Syncephalus: One head with a single face but four ears, and two bodies.

- Cephalothoracopagus: Bodies fused in the head and thorax. In this type of twins, there are two faces facing in opposite directions, or sometimes a single face and an enlarged skull.

- Xiphopagus: Two bodies fused in the xiphoid cartilage, which is approximately from the navel to the lower breastbone. These twins almost never share any vital organs, with the exception of the liver. A famous example is Chang and Eng Bunker.

- Ischiopagus: Fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle. These twins have four arms; one, two, three or four legs; and typically one external set of genitalia and anus.

- Omphalo-Ischiopagus: Fused in a similar fashion as ischiopagus twins, but facing each other with a joined abdomen akin to omphalopagus. These twins have four arms, and two, three, or four legs.

- Parapagus: Fused side-by-side with a shared pelvis. Twins that are dithoracic parapagus are fused at the abdomen and pelvis, but not the thorax. Twins that are diprosopic parapagus have one trunk and two faces. Twins that are dicephalic parapagus have one trunk and two heads, and have two (dibrachius), three (tribrachius), or four (tetrabrachius) arms.

- Craniopagus parasiticus: Like craniopagus, but with a second bodiless head attached to the dominant head.

- Pygopagus (Iliopagus): Two bodies joined at the pelvis.

- Rachipagus: Twins joined along the dorsal aspect (back) of their bodies, with fusion of the vertebral arches and the soft tissue from the head to the buttocks

There are two categories of craniopagus twins:

- Partial Although partial craniopagus is less common than total, it is still a division of craniopagus twins that is worth exploring. This type of twinning is defined as having limited surface area involvement, with either intact crania or cranial defects. In other words, it is a defect of the cranial "coverings." In partial craniopagus twins, the unions are usually frontal and less commonly occipital and vertical. Angular frontal junctions occur when the two twins are joined at any part of the forehead. Occipital twins are joined at the occipital lobe in the back of the head and vertical are joined on the top of the head and usually face opposite directions. The junctional diameter is often smaller in partial forms and occasionally an incomplete layer of bone may be present between the twins. Each child maintains independent calvarial convexities except at the common area of skull junction. The dura of both children may be intact or deficient and cortical gyri may interdigitate. Additionally shared dural venous sinuses is usually absent or if it is present it is negligible. These twins usually undergo successful separation and both twins may live to lead normal lives.

- Total Total craniopagus twins are defined as sharing extensive surface area with widely connected cranial cavities. Among total craniopagus twins, there are four main categories which are then further divided into several subcategories. Frontal, the first category, are when twins are facing each other with the axis of the bodies forming an acute angle. Temporoparietal craniopagi are joined immediately above the external auditory meatus. The third division is the occipital anomaly where the twins are connected in the occipital lobe causing the twins to face away from each other. The final variant is the parietal craniopagus which occurs when twins fuse at the vertex with the axis of the twins forming an obtuse angle. This category is perhaps the most important, or most interesting because the craniums of the two twins share the most veins, lobes and circuitry and is often described as one brain shared by two individuals.

- "Type 1": both children face in the same general directional axis so that the angle between twins is less than 40 degrees. These twins show relatively symmetric superior bi-parietal or vertex compressional flattening.

- "Type 2": both children face opposite directions so that the deformity shows an axial rotation between 140-180 degrees.

- "Type 3": in this variety axial rotation is intermediate between the first two types with a rotation of being between 40 and 140.

"'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.

Polycephaly is the condition of having more than one head. The term is derived from the Egyptian stems "poly" (Greek: "πολύ") meaning "many" and "kephalē" (Greek: "κεφάλη") meaning "head". A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.

Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.

In humans, there are two forms of twinning that can lead to two heads being supported by a single torso. In dicephalus parapagus dipus, the two heads are side by side. In craniopagus parasiticus, the two heads are joined directly to each other, but only one head has a functional torso. Survival to adulthood is rare, but does occur in some forms of dicephalus parapagus dipus.

There are many occurrences of multi-headed animals in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.

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

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.

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.

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.

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.

Foodborne illness (also foodborne disease and colloquially referred to as food poisoning) is any illness resulting from the food spoilage of contaminated food, pathogenic bacteria, viruses, or parasites that contaminate food, as well as toxins such as poisonous mushrooms and various species of beans that have not been boiled for at least 10 minutes.

Symptoms vary depending on the cause, and are described below in this article. A few broad generalizations can be made, e.g.: The incubation period ranges from hours to days, depending on the cause and on how much was consumed. The incubation period tends to cause sufferers to not associate the symptoms with the item consumed, and so to cause sufferers to attribute the symptoms to gastroenteritis for example.

Symptoms often include vomiting, fever, and aches, and may include diarrhea. Bouts of vomiting can be repeated with an extended delay in between, because even if infected food was eliminated from the stomach in the first bout, microbes (if applicable) can pass through the stomach into the intestine and begin to multiply. Some types of microbes stay in the intestine, some produce a toxin that is absorbed into the bloodstream, and some can directly invade deeper body tissues.

Foodborne illness usually arises from improper handling, preparation, or food storage. Good hygiene practices before, during, and after food preparation can reduce the chances of contracting an illness. There is a consensus in the public health community that regular hand-washing is one of the most effective defenses against the spread of foodborne illness. The action of monitoring food to ensure that it will not cause foodborne illness is known as food safety. Foodborne disease can also be caused by a large variety of toxins that affect the environment.

Furthermore, foodborne illness can be caused by pesticides or medicines in food and natural toxic substances such as poisonous mushrooms or reef fish.

Because liver cancer is an umbrella term for many types of cancer, the signs and symptoms depend on what type of cancer is present. Cholangiocarcinoma is associated with sweating, jaundice, abdominal pain, weight loss and liver enlargement. Hepatocellular carcinoma is associated with abdominal mass, abdominal pain, emesis, anemia, back pain, jaundice, itching, weight loss and fever.

The most frequent liver cancer, accounting for approximately 75% of all primary liver cancers, is hepatocellular carcinoma (HCC) (also named "hepatoma", which is a misnomer because adenomas are usually benign). HCC is a cancer formed by liver cells, known as hepatocytes, that become malignant. Another type of cancer formed by liver cells is hepatoblastoma, which is specifically formed by immature liver cells. It is a rare malignant tumor that primarily develops in children, and accounts for approximately 1% of all cancers in children and 79% of all primary liver cancers under the age of 15. Most hepatoblastomas form in the right lobe.

Liver cancer can also form from other structures within the liver such as the bile duct, blood vessels and immune cells. Cancer of the bile duct (cholangiocarcinoma and cholangiocellular cystadenocarcinoma) account for approximately 6% of primary liver cancers. There is also a variant type of HCC that consists of both HCC and cholangiocarcinoma. Tumors of the blood vessels (angiosarcoma and hemangioendothelioma, embryonal sarcoma and fibrosarcoma are produced from a type of connective tissue known as mesenchyme. Cancers produced from muscle in the liver are leiomyosarcoma and rhabdomyosarcoma. Other less common liver cancers include carcinosarcomas, teratomas, yolk sac tumours, carcinoid tumours and lymphomas. Lymphomas usually have diffuse infiltration to liver, but It may also form a liver mass in rare occasions.

Many cancers found in the liver are not true liver cancers, but are cancers from other sites in the body that have spread to the liver (known as metastases). Frequently, the site of origin is the gastrointestinal tract, since the liver is close to many of these metabolically active, blood-rich organs near to blood vessels and lymph nodes (such as pancreatic cancer, stomach cancer, colon cancer and carcinoid tumors mainly of the appendix), but also from breast cancer, ovarian cancer, lung cancer, renal cancer, prostate cancer.

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.