The main risk factor for Alzheimer's disease is increased age. There are also genetic risk factors for Alzheimer's disease. Most patients develop Alzheimer's disease after age 70. However, 2%-5% of patients develop the disease in the fourth or fifth decade of life (40s or 50s). At least half of these early onset patients have inherited gene mutations associated with their Alzheimer's disease. Moreover, the children of a patient with early onset Alzheimer's disease who has one of these gene mutations has a 50% risk of developing Alzheimer's disease.

There is also a genetic risk for late onset cases. A relatively common form of a gene located on chromosome 19 is associated with late onset Alzheimer's disease. In the majority of Alzheimer's disease cases, however, no specific genetic risks have yet been identified.

Early symptoms: memory problems; mild personality changes, such as less spontaneity, apathy, and a tendency to withdraw from social interactions;

Late symptoms: problems in abstract thinking and in other intellectual functions; confusion & disorientation; unable to engage in conversation, erratic in mood, uncooperative, and lose bladder and bowel control; Incapability of caring for themselves;

Hope this helps.

Alzheimer's disease, first described by Alois Alzheimer in 1906, is a disease that kills off brain cells, disrupts synapses, and causes the brain itself to shrink severely (in advanced cases). In severe cases a person loses the ability to form memories or access old memories, the ability to relate to their surroundings, and the ability to care for themselves. While some people can survive 20 years or more after diagnosis, it is the 7th leading cause of death in the US.

For many more details, including a "brain tour" that describes how the brain works and how Alzheimer's disease affects it, see http://www.alz.org/alzheimers_disease_4719.asp .

http://www.alz.org/alzheimers_disease_causes_risk_factors.asp describes the risk factors as including age, family history (the more members of the family who have it, the higher the risk is but it is not a certainty except in "familial Alzheimer's disease" which is extremely rare), heredity, head injuries, heart illness, and general healthy aging (or lack thereof). As to this last, the site says:

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Other lines of evidence suggest that strategies for overall healthy aging may help keep the brain healthy and may even offer some protection against developing Alzheimer’s or related diseases. Try to keep your weight within recommended guidelines, avoid tobacco and excess alcohol, stay socially connected, and exercise both your body and mind.
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Useful tip :
Garlic helps in fighting this disease.
Another herb Brahmi (available mostly in India) is extremely good.
Meditation is also a powerful way and is very good for the brain.
Even exercising helps.

Alzheimers Disease is a tragic that robs victims and their families of years of happiness by slowly destroying the mind and personality of the person afflicted. Though there is no cure yet, there are drugs available to treat symptoms and delay progression of Alzheimer's if it is diagnosed early. A new test called a bio-barcode assay, which may be able to identify a certain protein in spinal fluid associated with the disease, is being studied to determine its usefulness in the early detection of the disease.