David Cameron's son Ivan has died of a condition which some doctors refer to as Ohtahara syndrome, a very rare form of epilepsy which also exhibits symptoms akin to those of cerebral palsy.
Cameron's son died at the age of six
What exactly is it?
Ohtahara syndrome is either inherited or may develop around the time of birth, possibly as a result of brain damage. It is characterised by seizures which start in the first days of life.
Some babies have as many as 100 of these seizures every day.
In the case of Ivan Cameron, his parents noted soon after his birth in 2002 that something was wrong: he had occasional spasms and seemed sleepy.
After extensive tests, they were informed of the diagnosis and warned he would be unlikely to ever walk or talk.
As well as being excessively sleepy, babies with Ohtahara syndrome are often very floppy.
They usually go on to develop stiffness in their limbs - similar to children with cerebral palsy - which gives them severe impairments.
They tend to make very little developmental progress and remain totally dependent on others.
There is still some debate as to whether it should be classified as a distinct condition, as Dr S Ohtahara did in 1976, or whether it should be seen as severe early onset epilepsy with a range of differing symptoms.
How rare is it?
It is thought that Ohtahara syndrome affects about 0.2% of children with epilepsy, which in turn affects about 0.5% of the population. Boys are thought to be slightly more affected than girls.
What kind of care does a child with the condition need?
Round-the-clock attention. "It is a 24 hour commitment," says Diane Smyth, a consultant paediatrician at St Mary's Hospital who was involved in Ivan's care.
"It is very complex and very demanding on the family and the siblings. Many professionals are needed to help optimise the skills of the child and ensure the quality of life is as good as possible."
Is it always fatal?
The mortality rate is very high, according to Professor Brian Neville at UCL's Institute of Child Health. As many as half will die in infancy or childhood.
What is known as Sudden Unexplained Death in Epilepsy - or Sudep - affects a significant minority of all people with epilepsy, and this is a common cause of death for those with the syndrome.
The risk of Sudep may be reduced with medication to control seizures, but Ohtahara syndrome is often resistant to many of the anti-epileptic drugs available. Older medications such as phenobarbitone as still the preferred choice.
Some babies with a clear abnormality in the brain may be suitable for surgery to remove the affected area, but this is unusual.
Breathing difficulties are often associated with the condition, and those affected are more susceptible to illness.
Pneumonia is not uncommon, and children often die within the first two years of life as a result of such chest infections.
What are the risks of having another child with the condition?
Lack of research means it is very hard for doctors to give parents an accurate assessment of what this risk would be. Some put it at 5% - but this may be based on all forms of epilepsy as a whole. Very few cases are known of families with more than one child with the condition.
Speaking to the BBC in 2006, David Cameron spoke about his fears when he and his wife had their subsequent children, Nancy and Arthur.
"When they were born we were watching them like hawks to see if everything was all right," he said on Desert Island Discs.
"Instead of that great elation you should have at the birth of a child, the first week, two weeks were very tense indeed."