Many different things can affect brain function, from infection, to trauma, to metabolic imbalances, but only a few of them are deadly. Most of these deadly diseases are equal opportunity, infecting just as many people from First World countries as they do from underdeveloped nations. Only a few diseases are specific to one region or location in the world, while the rest of them can strike anyone at anytime.
Prions are a type of highly infectious protein also known as transmissible spongiform encephalopathies (TSE). These proteins can infect different species and are always fatal. The CDC states that this disease kills by causing abnormal folding of cells in the brain and a rapid loss of cells, along with a decline in nervous function. Infection is caused by eating an infected animal. In addition, the Merck Manual states that the protein disease can be inherited or occur spontaneously. Creutzfeldt-Jakob disease is the most common form in humans. This is also the same as "Mad Cow" disease, but it can infect sheep, deer and elk in the same way. Kuru is another name for this disease that affects humans, but this name is given only to cannibalistic populations that become infected from eating the brains of an infected deceased human.
The Naegleria fowleri species of amoeba or parasite is found in infected warm freshwater and soil. An amoeba is a single-celled organism and can live outside the body, so it does not need the human host to survive. Death from this infection is one of the most rapid of the brain diseases, causing failure to thrive within three to seven days, according to the CDC. The amoeba often enters the brain through the nose while swimming and starts to destroy brain tissue immediately. Even with treatment, most of those infected die. While it is rapidly fatal, it is very rare. In the United States the CDC states there have only been 33 reported cases from 1998 to 2007.
This is a rare inherited brain disease that occurs in one out of every 10,000 people in the U.S. The Huntington's Disease Society of America states that an affected parent has a 50 percent chance of passing the gene on to the next generation. Often this disease starts in people between the ages of 30 to 50 and is a progressive loss of the basic autonomic functions in the brain, such as movement, swallowing and breathing. There is no cure for this disease and no set rule as to when it will start. Death is usually caused by the lack of ability to care for oneself anymore, infections, and choking due to the inability to swallow.
Dementia has many forms of disease, but all of them involve the degeneration of brain tissue. Helpguide.org states the most common form of dementia is Alzheimer's disease. Dementia is usually a long process of 20 or more years. In the late stages of the disease, those affected with dementia lose the ability to talk, swallow or walk. It is a rather vegetative state, and death usually results because the victim cannot communicate discomfort from an illness, and infection occurs as a result of lack of treatment, or the victim can no longer care for herself.
Rabies is a swiftly moving virus that can affect a number of mammals, including humans. Once someone is infected with rabies, this disease is 100 percent fatal without treatment. The virus itself infects the brain and central nervous system. The CDC states that once someone is infected, death occurs within days of the onset of symptoms. Initial symptoms of rabies are very similar to those of many other diseases, like the cold or flu, so it may be difficult to distinguish at first. An infected human can also infect another human with only a bite.