context: Prion diseases are a group of different conditions that affect your brain and nervous system. They can cause severe dementia or problems with body control that get worse very quickly. They're extremely rare -- the U. S. has only about 350 cases of prion diseases each year. Prions are tiny proteins that, for some reason, fold over in a way that damages healthy brain cells. You can have them for many years before you notice any symptoms. Even though prion diseases cause dementia, Alzheimer's disease is not in this group. Proteins that don't work the way they should and damage brain cells are to blame for both diseases, but the diseases involve different genes and proteins. Alzheimer's symptoms may get worse slowly and gradually over a few years. Prion diseases get worse very quickly. But like Alzheimer's, there is no cure for prion diseases. Prion diseases are also called transmissible spongiform encephalopathies, or TSE, diseases. Both humans and animals can get them. Prion diseases that affect people include: Creutzfeldt- Jakob disease ( CJD) Variant Creutzfeldt- Jakob disease ( VCJD) Gerstmann- Straussler- Scheinker syndrome Fatal familial insomnia Kuru Variably protease-sensitive prionopathy Creutzfeld- Jakob disease is the most common. It usually affects people around age 60. VCJD can affect younger adults. Signs of prion diseases include sudden changes in your mood, memory, and movement, such as: Anxiety or depression Balance problems Behavior or personality changes Dementia Memory loss Muscle control loss, like sudden jerks or twitches Seizures Slurred speech Trouble swallowing Unsteady walk Vision problems Prions are tiny proteins in your brain that don't act the way they should. They fold over the wrong way, spread, and then cause other proteins to form the same incorrect shape. These badly shaped prions build up and form clumps in your brain. Then they kill neurons, or brain cells that control your memory, balance, and movement. Your immune system can't fight them off, so more healthy cells die. Most people get prion diseases like CJD for no apparent reason. Around 15% of people get prion diseases because they have a problem gene called PRNP. It can run in families. Very rarely, people get prion diseases from infections, such as from donated tissue in a transplant operation or from unclean tools your surgeon used. It's possible to get VCJD if you eat beef infected with bovine spongiform encephalopathy, or mad cow disease. When mad cow disease broke out in cattle in Europe in the 1990s, a small number of people developed VCJD and died. Four people also got VCJD from blood transfusions when the donor was infected. To lower the risk of VCDJ, countries changed how cattle are fed and how donated blood is collected or treated. It's hard to diagnose prion diseases. If you have symptoms like dementia, your doctor might first rule out other possible causes. The following tests may show if your symptoms are caused by a stroke, brain tumor, or inflammation: Spinal tap (also called lumbar puncture): A doctor puts a needle between two of your vertebrae (bones in your back) to take a sample of the fluid around your brain and spinal cord. MRI (magnetic resonance imaging): Powerful magnets and radio waves are used to make detailed images of your brain. CT scan (computerized tomography): Several X-rays taken from different angles are put together to show a clearer picture of your brain. Your doctor can only be certain you have a prion disease if she takes a sample of your brain tissue (called a brain biopsy). But this is a dangerous operation, so it's only done if your doctor thinks you might have another disorder that could be treated differently. For now, treatments for prion diseases only ease your symptoms. They include pain drugs, antidepressants, sedatives, or antipsychotic drugs. You also may need a catheter to help drain urine or feeding tubes to help you eat.