The nerve cells that die and lead to the development of Parkinson's are responsible for producing a chemical known as dopamine.
Dopamine allows messages to be sent to the parts of the brain that co-ordinate movement.
With the loss of dopamine-producing nerve cells, these parts of the brain are unable to work normally, causing symptoms of Parkinson's to appear.
The level of dopamine then continues to fall slowly over many years, causing symptoms to further develop and new symptoms to appear.
It's very rare for people to pass on Parkinson's to their children.
It is estimated that only a very small number of people may have an increased risk of Parkinson's linked to their genes.
There is some evidence that environmental factors (toxins) may cause dopamine-producing neurons to die, leading to the development of Parkinson's.
The term 'environment' refers to the world around you and the pathogens (viruses and bacteria), toxic chemicals and heavy metals that occupy it.
In particular, there has been a great deal of speculation about the link between the use of herbicides and pesticides and the development of Parkinson's.
Read our research news for more on the latest developments.
What we know so far
We're still building a picture of why people get Parkinson's, but understanding the pieces of this complex puzzle will help us unlock better treatments and one day a cure.
Watch our short animation to find out what we know so far about the causes of Parkinson's.