Alzheimer's disease is a progressive and irreversible brain disorder that affects memory, thinking, and behavior. It is the most common cause of dementia, a syndrome characterized by a decline in cognitive function that interferes with daily activities. According to the World Health Organization (WHO), there are currently around 50 million people worldwide living with dementia, and this number is expected to triple by 2050. Epidemiology refers to the study of the distribution and determinants of diseases in populations. Several risk factors have been identified for Alzheimer's disease, including age, family history, genetics, lifestyle factors such as diet and exercise, and certain medical conditions such as hypertension and diabetes. Alzheimer's disease is most commonly diagnosed in people over the age of 65, although early-onset Alzheimer's can occur in people in their 40s or 50s.