It arrives by stealth. It takes root imperceptibly, but soon you'll feel a twitch. By then motor neuron disease is laying waste to cells that control essential voluntary activity, even the ability to breathe and swallow.
Ultimately it brings death. MND afflicts only a small part of humanity. That perhaps explains why scientists who are trying to shed light on it do so almost obsessively.
The disease has revealed some patterns. It doesn't recognise borders. It can afflict men in their 40s like Springbok rugby star Joost van der Westhuizen, academics in their 50s like Dale Ranchhod, entrepreneurs in their 30s like Scotsman Euan MacDonald, and, most famously, theoretical physicist and cosmologist Stephen Hawking.
Men and women of sound mind become trapped inside their bodies by a condition that gradually strips away their quality of life.
In the US it is known as amyotrophic lateral sclerosis, or Lou Gehrig's disease, named after a New York Yankees baseball star who died aged 37.
When Van der Westhuizen visited the US he learnt that Gehrig broke his hand 23 times. He almost cheerily observed: "I broke my nose 16 times."Breaking bones was not all they had in common.
Van der Westhuizen died in February last year, an adversary of MND until the end. He had been on a mission to bring wider awareness of the disease, most notably through the Centre for Neurodegeneration that takes his name. With it he hoped efforts to fight the disease would be more concentrated.
Van der Westhuizen, despite limited speech, did not go quietly. On a crisp July morning his dream to have a facility dedicated to the treatment of patients with the disease was opened at the Chris Hani Baragwanath Academic Hospital. It is a tribute to private-public collaboration.
"Patients now have regular access to therapy across a range of disciplines, including medical care, physiotherapy, occupational therapy, speech therapy and social workers," said Gauteng health MEC Gwen Ramokgopa.
"This initiative is a good example of how the collective efforts and focus of the private and public sectors can deliver world-class solutions and make a meaningful difference to patients and the healthcare system at large," said Stavros Nicolaou, a senior executive at Aspen Pharmacare.
Andre Mochan, who heads the clinic, had more sobering news. "We can't cure the disease, but we can slow it down. We can accompany patients on this journey."
He said the idea for the clinic took root when he accompanied Van der Westhuizen to a research facility in Edinburgh in November 2013.
The Soweto facility has operated since late 2014 and has a face, and funding. Challenges, however, remain; sufferers need transport to get there.
Mochan said MND operated in the shadows. "There are many potential explanations of how it starts but the more theories you have, the more you actually realise that you don't know."
How it affected a sporting icon who embodied vitality and physical prowess remains a mystery."Many sports people are affected by this ... certainly more than in the average population," said Mochan. "There has been a study done with Italian soccer players. There is an interesting connection with athletes."
Extensive research on Italian footballers was done by Professor Adriano Chio at the University of Turin. He concluded that from the 1970s, 41 professional players had contracted the disease. This meant that a person was seven times more likely to suffer MND playing professional football in Italy than, say, a farm labourer in Piedmont, or a factory worker at Fiat.
"One of the theories is that there is something that exposes them on the sports field to some sort of environmental agent, or a fertiliser that is used on the field, or something along those lines. That agent hasn't been found," says Mochan. "The other theory is that sports stars are more likely to be afflicted because they overexercise their muscles, so it could be linked to the muscular system.
"There are other theories that go into environmental toxins. There are others that look into more genetic mechanisms whereby it is a built-in clock in your own body that switches off those particular cells for some unknown reason. That has something to do with your genes.
"The bottom line is there are many theories and we haven't put the puzzle together.
"It is probably multi-factorial. There is quite a lot of evidence emerging from genetic research that there are certain parts of our genes that might predispose us to the disease and its acceleration. So there is probably an interaction between genetic factors that we have within ourselves and environmental factors. There is probably a combination of those factors," says Mochan.
It has been established that 10% become sufferers because they are genetically predisposed. It is more likely to attack men than women.
"This is a monster we are dealing with. It is one of the worst diseases that can be bestowed on mankind. Your nerves die for no apparent reason. It is relentless, progressive paralysis. Then your speech goes as well as your breathing muscles. The patient is fully aware of what's happening. Then you die."
Five South African rugby players have contracted the disease since 2011. Tinus Linee, a former teammate of Van der Westhuizen, died of MND at the age of 45 in 2014.Former Springbok hardman Andre Venter has transverse myelitis, inflammation of the spinal cord, which is considered part of the MND family.
The question has been asked whether there was a link between those who suffered head trauma and MND.
"That link hasn't been established with those particular sports where there is repeated concussion or brain trauma. We are watching those things epidemiologically. That correlation hasn't been made," said Mochan.
He says there is a drive to destigmatise the disease and to educate people. Sufferers in townships are often ostracised by those who believe they are HIV-positive. Van der Westhuizen, who had an aggressive form of the disease, didn't have to fight that battle. He could confront the beast head-on. The more people know, the better.
In Glory Game, a DVD chronicling his fight against the disease, Van der Westhuizen is asked in a radio interview how difficult it is to accept his fate. "Initially you ask 'why me?'. Actually, 'why not me?'."
Later he says: "Maybe having the disease was my destiny. If it is, I'm happy. It's quite simple. It is the way you react to the disease that will make a difference.
"The two things we take for granted are time and our health. When we don't have it, it's usually too late."..