In Australia, one in 350 people have the neurodegenerative disorder, according to Parkinson's Australia.
It is caused by a loss of brain cells that produce a chemical messenger called dopamine. Symptoms differ from case to case, but often include a tremor or fine shake while the person is at rest, rigidity of muscles, slowness of movement and unsteady balance.
Other possible symptoms can include memory loss. Earlier this year, Connolly admitted he had started to forget his lines during performances.
There is no cure for Parkinson's and scientists have been unable to work out why people get the condition.
Symptoms can be controlled using a combination of drugs, therapies and occasionally surgery, but often more care and support are needed as the disease progresses.
Steve Ford, chief executive at Parkinson's UK, says diagnosis can be a "positive" experience for people.
"Often people when they are diagnosed with Parkinson's will say, 'Now I realise I've probably had it for several years'," he says.
"They might have been living with a tremor in their fingers or a stiffness, which is why diagnosis can be a positive and they can start treatment for a tremor or slowness of movement. "It is important to say that the treatment can deal with some of the symptoms but not the underlying causes and Parkinson's is a progressive illness, so as you go on more of the cells in the brain die and it gets worse although the rate at which it gets worse differs in individuals."
Ford says physiotherapy sessions and speech and language therapy can help people to deal with the symptoms.
Actor Bob Hoskins announced his retirement last year after being diagnosed with Parkinson's.
The disease was identified by - and named after - Dr James Parkinson, who in 1817 wrote An Essay On The Shaking Palsy, which established it as a recognised medical condition. It was announced on Monday that Connolly has been diagnosed with early symptoms of Parkinson's disease and had undergone successful surgery for prostate cancer.
