Today is World Alzheimer's Day.
Alzheimer's disease is a condition which affects the brain. It is the most common cause of dementia - also a condition which affects people's brains - but not the only one.
Dementia can make it difficult to remember things. People might forget who family members are. They might get lost or they may forget what everyday objects are called.
It can make people behave in a way that they've never done before. They may become very upset, sad or angry - and find it difficult to deal with these feelings.
Alzheimer's disease and dementia can affect people of all different ages, but usually it affects much older people.
According to the Alzheimer's Society, there are around 850,000 people in the UK with dementia.
One in 14 people over 65 will develop dementia, and the condition affects one in six people over 80.
The number of people with dementia is increasing because people are living longer. It is thought that by 2025, the number of people with dementia in the UK will have gone up to around one million.
It can be very difficult for families to deal with dementia. People they love behave in unusual ways that they may never have done before.
It's upsetting for families if the person with the disease doesn't recognise their own children or grandchildren.
A person with dementia can often feel very confused and it's tough for the family to make them feel better.
There is a lot of support out there. If you're feeling worried or upset about dementia in your family, the most important thing is to talk to an adult you trust.
Don't try and deal with everything by yourself. Lots of special groups, charities and people are out there to help you.