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This is SCIENCE IN THE NEWS, in VOA Special English. I'm Bob Doughty.

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And I'm Faith Lapidus. Today we tell about Alzheimer's disease. One century after its discovery, the cause of the disease is still unknown.

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In November of nineteen ninety-four, Ronald Reagan wrote a letter to the American people. The former president shared the news that he had Alzheimer's disease. Mister Reagan began what he called his journey into the sunset of his life. That ten year journey ended on June fifth, two thousand four, at the age of ninety-three.

In his letter, America's fortieth President wrote about the fears and difficulties presented by Alzheimer's disease. He said that he and his wife Nancy hoped their public announcement would lead to greater understanding of the condition among individuals and families affected by it.

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Ronald Reagan was probably the most famous person to suffer from Alzheimer's disease. In the United States, about four million five hundred thousand people have the disease. Many millions more are expected to have it in years to come.

Doctors describe Alzheimer's as a slowly increasing brain disorder. It affects memory and personality -- those qualities that make a person an individual. There is no known cure. Victims slowly lose their abilities to deal with everyday life. At first they forget simple things, like where they put something or a person's name.

As time passes, they forget more and more. They forget the names of their husband, wife or children. Then they forget who they are. Finally, they remember nothing. It is as if their brain dies before the other parts of the body. Victims of Alzheimer's do die from the disease, but it many take many years.

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Alzheimer's disease is the most common kind of a disability or mental sickness called dementia. Dementia is the loss of thinking ability that is severe enough to interfere with daily activities. It is not a disease itself. Instead, dementia is a group of signs of some conditions and diseases.

Some kinds of dementia can be cured or corrected. This is especially true if they are caused by drugs, alcohol, infection sight or hearing problems, heart or lung problems or head injury. Other kinds of dementia can be corrected by changing levels of hormones or vitamins in the body. However, brain cells of Alzheimer's victims die and are not replaced.

Victims can become angry and violent as the ability to remember and think decreases. Often they shout and move about with no purpose or goal. Media reports often tell about older people found walking in places far from their homes, not knowing where they are or where they came from. Generally, these people are suffering from Alzheimer's disease.

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Alzheimer's disease generally develops differently in each person. Yet some early signs of the disease are common. Often, the victims may not recognize changes in themselves. Others see the changes and the struggle to hide them.

Probably the most common early sign is short-term memory loss. The victim cannot remember something that happened yesterday, for example. Also, victims of the disease have increasing difficulty learning and storing new information. Slowly, thinking becomes much more difficult. The victims cannot understand a joke, or cannot cook a meal, or perform simple work.

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Another sign of Alzheimer's disease is difficulty solving simple problems, such as what to do if food on a stove is burning. Also, people have trouble following directions or finding their way to nearby places.

Another sign is struggling to find the right words to express thoughts or understand what is being discussed. Finally, people with Alzheimer's seem to change. Quiet people may become noisier and aggressive. They may easily become angry and lose their ability to trust others.

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Alzheimer's disease normally affects people more than sixty-five years old. But a few rare cases have been discovered in people younger than fifty. The average age of those found to have the disease is about eighty years old.

Alzheimer's is found in only about two percent of people who are sixty-five. But the risk increases to about twenty percent by age eighty. By ninety, half of all people are found to have some signs of the disease.

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Alzheimer's disease affects people of all races equally. Yet women are more likely to develop the disease than men. This is partly because women generally live longer than men.

There is no simple test to show if someone has Alzheimer's disease. Doctors who suspect Alzheimer's must test the patient for many other disabilities first. Alzheimer's is considered the cause if the tests fail to show the presence of other disabilities. The only way to tell for sure if a person has Alzheimer's is to examine the victim's brain after death.

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Recently, scientists reported progress in efforts to identify persons who will develop Alzheimer's disease. For example, one study examined brain and spinal cord fluid from sixty-eight people. It found twenty-three proteins that showed evidence of the disease. Study organizers said the protein test was correct in about ninety percent of patients involved in the study. The results were confirmed with brain examinations after the patients died.

Another study found evidence of Alzheimer's by using a chemical known as F-D-D-N-P. This study used a process called positron emission tomography to make brain images of eighty-three adults. American scientists said the test was ninety-eight percent correct in showing differences between Alzheimer's and normal memory problems.

Scientists say all these results must be repeated with larger groups of patients. But they said that being able to find the presence of the disease in such ways would make early treatment possible.

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In his book "The Notebook," Nicholas Sparks calls Alzheimer's disease, "a thief of hearts and souls and memories." British writer Iris Murdoch died of the disease. She said it was a dark and terrible place.

It has been more than a century since a German doctor, Alois Alzheimer, told about a dementia patient whose brain was studied after death. Her brain had sticky structures and nerve cells that appeared to be mixed together. Later studies showed these tangled nerves are made of a protein called tau. The tau protein changes so that it sticks together in groups. The sticky structures were shown to be amyloid plaques.

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Scientists are still not sure what causes Alzheimer's disease. The leading theory blames amyloid plaques. Reports say about one hundred different drugs are being tested to treat or slow the progress of the disease.

Some American scientists have found a way to reduce amyloid plaque development. Researchers in New York say they reduced the amount of amyloid protein in the brains of mice by fifty percent. They say they did this by stopping interaction between amyloid and a protein known as apo E. Apo E moves cholesterol and other fats around the brain.

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Not all scientists are sure that amyloid plaques cause Alzheimer's disease. Some say the plaques could be an effect of the disease, not the cause. Reports say some people who die of Alzheimer's do not have any plaques in their brains. Others who have the sticky structures showed no signs of Alzheimer's.

Many scientists now say doctors are considering other possibilities. These include studies of enzymes that act on proteins to produce the plaques, and using antibodies against amyloid. Yet amyloid and enzymes are important for health and scientists do not want to destroy them completely.

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Other scientists are working with a gene called apoE4. Scientists in nineteen ninety-three discovered that its presence increases the chance of developing Alzheimer's disease. They now say that apoE4 is present in fifty to seventy percent of the patients with the disease.

Some scientists are attempting to change the protein that apoE4 makes. Others are working to block an enzyme that divides the apoE4 protein into different pieces that kill nerve cells.

Many more studies are being done to find the cause and treatment for Alzheimer's...a disease that continues to affect millions of people around the world.

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SCIENCE IN THE NEWS was written by Nancy Steinbach. Our producer was Brianna Blake. This is Bob Doughty.

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And this is Faith Lapidus. Join us again next week for more news about science in Special English on the Voice of America.