Alzheimer's disease
People who have Alzheimer's disease slowly lose their memory and ability to think clearly. As the disease gets worse, they get more confused and start acting differently. Several changes happen in the brain that stop it working properly. Small lumps called amyloid plaques grow in the parts of the brain used for memory and thinking. And bundles of twisted threads called 'neurofibrillary tangles' form inside brain cells. These stop brain cells communicating with each other, and they can cause cells to die. Also, in Alzheimer's disease, the brain does not have enough chemical messengers (neurotransmitters), and holes or gaps appear where brain cells have died.

Lewy body dementia
People with Lewy body dementia slowly lose their memory and ability to think clearly. Most people also get symptoms of Parkinson's disease. This means they may move more slowly and become rather stiff and rigid in the way they do things. People with this type of dementia get small, round clusters of proteins inside their brain cells. These are called Lewy bodies. They stop important chemicals in the brain from working normally. Lewy body dementia can also cause small lumps in the brain called amyloid plaques. They're made of protein and bits of dead cells. The lumps grow in the parts of the brain used for memory and thinking. They may stop messages from passing between brain cells.Lewy body dementia is the second most common cause of dementia after Alzheimer's disease.

vascular dementia
This type of dementia happens when blood vessels near your brain get damaged, so your brain doesn't get enough blood. Some of the cells in your brain die, which causes symptoms such as confusion and memory problems. Vascular dementia can happen suddenly if you have a stroke. Or it can come on gradually if you have a series of little strokes (this is called multi-infarct dementia).