There are certain abnormalities found in the brains of people with Alzheimer's disease, but experts don't know whether these cause the disease or are a result of the disease.

Neurofibrillary tangles are twisted structures formed from part of the nerve.

Beta amyloid is a sticky substance that clumps to form structures called plaques. Plaques and tangles damage the healthy brain cells that surround them, causing the brain to waste away and shrink. They are both found in the brains of people with Alzheimer's disease.

Another characteristic of Alzheimer's disease is that brain cells produce reduced amounts of chemicals, known as neurotransmitters, that are required for communication between nerves. These chemicals include acetylcholine, serotonin, and norepinephrine.

Risk Factors for Alzheimer's Disease

Factors that may increase the risk for Alzheimer's disease include:

Age. The risk of developing Alzheimer's disease increases with age. According to the Alzheimer's Association, 10% of all people over the age of 65 have Alzheimer's disease, and as many as 50% of people over age 85 have it.
Gender. Alzheimer's disease affects women more frequently than men.
Family history. Fewer than 1% of Alzheimer's disease cases are inherited, mostly those with onset before age 65.
Down syndrome . People with Down syndrome often develop Alzheimer's disease in their 30s and 40s, although the exact reason is not known.
Head injury. Some studies have shown a link between Alzheimer's disease and a significant head injury.
Environmental toxins. Some researchers suspect that increased exposure to certain substances, such as aluminum, may make a person more susceptible to Alzheimer's disease.
Low education level. Although the reason is not clearly understood, some studies have shown that low education levels can be related to an increased risk for Alzheimer's disease.
Other factors. Research also suggests that high cholesterol levels and high blood pressure -- factors linked to heart disease and stroke -- may also increase the risk for developing Alzheimer's.