Most transmissible spongiform encephalopathies (TSE) tend to occur in older individuals. According to the National Institutes of Health, there is only one case of TSE per million people, worldwide, each year. However, according to the Centers for Disease Control and Prevention (CDC), the incidence of Creutzfeldt-Jakob disease (CJD), the most common form of TSE, increases to about 3.4 cases per million people over the age of 50.

TSE can be caused by a number of different mechanisms, including:

• Sporadic disease. In most cases, TSE occurs in individuals with no known risk factors. It is thought that this occurs due to a spontaneous change in the prion proteins. Some researchers believe that proteins are frequently becoming misfolded as they are created, but that the body breaks these proteins down before they cause problems. However, some may be “missed,” prompting proteins they come into contact with to misfold as well, leading to TSE. According to both the National Institute of Neurological Disorders and Stroke (NINDS) and the CDC, about 85 percent of CJD cases occur in this manner.
  
  
• Hereditary disease. Some cases of TSE may also be hereditary. According to the CDC, about 5 percent  to 15 percent of all cases of CJD are inherited. Genes affect all aspects of the human body. A change, or mutation, in a gene can cause any number of problems. Some gene mutations can lead to TSE, apparently by causing certain proteins to misfold, becoming infectious prions. This abnormal prion protein can be detected using genetic testing, but not all people with the gene develop a TSE.
  
  A genetic change may occur spontaneously in an individual without any known risk factors, or a mutated gene can be passed down by parents. These hereditary forms of TSE are inherited in an autosomal dominant fashion. This means that only one parent needs to pass the gene to the child.
  
  
• Foodborne contamination. The only human TSE positively linked to food contamination is variant Creutzfeldt-Jakob disease (vCJD). It is generally accepted that this infection occurs when food contaminated with bovine spongiform encephalopathy (BSE), also known as mad cow disease, is consumed. In theory, when the infectious prion responsible for BSE is consumed, it is absorbed into the person’s body, where it may cause certain proteins to misfold, creating more infectious prions. However, because the cow and the human are different species, this is not as efficient a transmission process as if the prion had come from another human. Therefore, the potential for a TSE to develop is reduced due to this “species barrier.”
  
  Not all parts of a cow infected with BSE are equally risky to consume. High-risk tissues include certain intestinal tissues, nerve tissues and tissues likely to be contaminated by nerve tissues (e.g., brain, eyes, tonsils). The infectious agents do not appear to be present in milk or muscle meat. However, certain cuts of meat (e.g., t-bone steaks) may have a higher risk of contamination by nervous tissue. Although gelatin is derived from the hides and bones of cattle, the risk that the vCJD agent can be transferred through eating gelatin appears to be very low.
  
  The risk of acquiring vCJD by eating beef and beef products is small, even in areas with high numbers of BSE cases. According to the CDC, this risk in the United Kingdom is only about one in 10 billion servings. This low risk is largely due to public health control measures that have been put in place to stop the spread of BSE.
  
  
• Acquired disease. TSE can also be acquired from other humans. However, this is very rare. According to NINDS, less than 1 percent of all known cases of CJD have been acquired. TSE cannot be transmitted through the air or through casual contact with an infected person. However, it may be transmitted through infected tissue (e.g., brain or other nervous tissue, spinal cord fluid) and contaminated medical instruments (e.g., improperly sterilized electrodes in the brain). Certain transplant procedures, including dura mater (a tissue covering the brain) grafts and cornea transplants, can also transmit the diseases.
  
  When a TSE is transferred due to a medical procedure, it is known as an iatrogenic case. According to the CDC, no iatrogenic cases related to medical equipment have occurred since current routine medical sterilization procedures have been implemented.
  
  Pituitary growth hormone was once derived from the pituitary glands of cadavers. There have been cases of CJD transmission via this process. However, modern growth hormone used in America is synthetic and poses no threat of TSE transmission.
  
  There have also been concerns regarding the potential transmission of CJD through blood transfusions. While there is no evidence that this has ever occurred in humans, transmission of vCJD through blood may have occurred in the United Kingdom. In response to this potential risk, the Food and Drug Administration (FDA) and the American Red Cross have increased donor requirements to safeguard the American blood supply. For example, people who have lived for more than three months in a country or countries where BSE is common cannot donate blood in the United States.