British physicist Stephen Hawking was one of the most famous sufferers of motor neurone disease, known as amyotrophic lateral sclerosis (ALS) in the United States.
The rare condition normally claims the lives of those who have it within two to three years of diagnosis, making Hawking's five-decade fight to overcome the disease an extraordinary exception.
The neurodegenerative condition attacks the motor nerve cells in the brain and spinal cord, hampering their ability to communicate with muscles and control voluntary movements, leading to eventual paralysis.
Early symptoms of stiffness and muscle weakness worsen over time as victims gradually lose the ability to walk, speak and breathe.
The deadly condition is very rare, with two new cases per 100,000 people every year – most typically among individuals aged between 55 and 65.
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It became something of a household name in 2014 after the viral "Ice Bucket Challenge", which saw people upload videos of themselves pouring cold water over their heads in a bid to raise awareness about the disease.
There is currently no cure or treatment that halts or reverses ALS, though there are some options that can help manage symptoms.
The disease takes two main forms, according to the ALS Association in the United States.
The vast majority of people suffer from a "sporadic" version that can affect anyone, while up to ten per cent of cases in the US are inherited.
Military veterans are up to twice as likely to be diagnosed as the general public, for unknown reasons.
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US President Barack Obama presents the Presidential Medal of Freedom to British theoretical physicist Stephen Hawking during a ceremony in the East Room at the White House on August 12, 2009. Jewel Samad / AFP -
Pope Francis, left, meets with English theoretical physicist and cosmologist Stephen Hawking at the Vatican on November 28, 2016. Filippo Monteforte / AFP -
Stephen Hawking and his new bride Elaine Mason pose for pictures after the blessing of their wedding at St Barnabus Church on September 16, 1995. Russell Boyce / Reuters -
British scientist Stephen Hawking appears during the opening ceremony of the London 2012 Paralympic Games at the Olympic Stadium in east London. Leon Neal / AFP -
South Africa's former President Nelson Mandela, right, meets with British scientist Professor Stephen Hawking in Johannesburg. Denis Farrell / AFP -
British theoretical physicist professor Stephen Hawking gives a lecture during the Starmus Festival on the Spanish Canary island of Tenerife on September 23, 2014. Desiree Martin / AFP -
US President Bill Clinton and British professor Stephen Hawking watch a scene from "Star Trek the Next Generation" during a "Millennium Evening" at the White House in Washington, DC on March 6, 1998. Tim Sloan / AFP -
Famous physicist Stephen Hawking and his then wife Elaine smile on the opening day of the Frankfurt Book Fair 2005 in Frankfurt Main, Germany, on October 19, 2005. Boris Roessler / EPA -
British scientist Stephen Hawking and his daughter Lucy Hawking arrive on the red carpet for the 2015 British Academy Film Awards ceremony at The Royal Opera House in London on February 2015. Facundo Arrizabalaga / EPA -
British theoretical physicist professor Stephen Hawking gives a lecture entitled: "A Brief History of Mine" during the Starmus Festival on the Spanish Canary island of Tenerife. Desiree Martin / AFP -
Professor Stephen Hawking poses for photographers upon arrival for the Interstellar Live show at the Royal Albert Hall in central London on March 30, 2015. Joel Ryan / Invision /AP -
British physicist Stephen Hawking looks at a book of Chile's Nobel Laureate Pablo Neruda after a meeting with Chile's President Michelle Bachelet (unseen) at the Presidential Palace in Santiago on January 17, 2008. Victor Ruiz Caballero / Reuters
The average survival time for those affected by ALS is three years, according to the ALS Association.
Only five per cent of patients live for 20 years or more.
Researchers have said Hawking's exceptional longevity remains a mystery, though some have noted that the disease's progression varies by patient and could be governed by genetics.
Other famous victims of the disease include playwright Sam Shepard, who died in August 2017, Sesame Street co-creator Jon Stone and jazz musician Charles Mingus.
ALS is commonly referred to as "Lou Gehrig's disease" in a nod to the baseball legend, who is believed to have died from the illness in 1941.
