**2.1 Overview**

The term dementia encapsulates a number of separate diseases that clinically manifest as a progressive decline in cognitive function. Alzheimer's disease (AD) is the most common form of dementia accounting for approximately 60% of all cases. It is estimated that the current 36 million sufferers worldwide will increase to 115 million by 2050 (Alzheimer's Disease International, 2009). In countries like Australia, dementia will become the number one health cost by the middle of this century exceeding both heart disease and cancer (Access Economics, 2009). These surprising predictions result from the additive effects of an ageing world population, the duration of the disease and the extended period where the patient is totally dependent on caregivers. The societal trend in Western countries towards professional aged care services accounts for the majority of the current and predicted costs.

 More importantly, AD is a cruel disease that robs patients of their sense of self and places considerable physical and psychological strain on caregivers, who are often isolated from society themselves. As the currently available treatments only provide transient improvement, an AD epidemic will only be halted with the development of new therapies. It has been predicted that a novel therapeutic agent that delays disease onset and progression by just one year would result in nine million fewer cases by 2050 (Brookmeyer *et al.,* 2007).

The other approach to reducing the worldwide burden of AD is prevention. This would be based on either avoiding identified risk factors or augmenting the impact of protective factors. Unfortunately finding such factors through epidemiological studies has proved extremely difficult to date.

The completion of the human genome project in 2001 ushered in a dramatic increase in the technologies that have subsequently been applied to understanding the pathogenesis of complex disorders, like AD. Genome-wide association and expression studies are now commonplace in AD research creating a more immediate and dynamic research environment. The application of these technologies to clinical, pathological and epidemiological studies of AD is starting to bear fruit although effective treatments are not necessarily close at hand.
