First they told him he had aphasia. Then that he had frontotemporal dementia. Between those two findings sits an entire year in which Bruce Willis's family thought the problem was his hearing, his age, the fact that the man had withdrawn into himself. That isn't a rare story - it's almost the rule with this illness.

Frontotemporal dementia is a group of neurodegenerative diseases in which neurons in the frontal and temporal parts of the brain gradually die. Those are precisely the parts that hold language, behaviour, emotional control, personality and the capacity to decide. According to the Spanish Society of Neurology, it's the third most common cause of neurodegenerative dementia, right after Alzheimer's and dementia with Lewy bodies. It affects around 0.2 to 0.3 percent of people over 65, but most often begins between 50 and 70 - at the age when a person is considered to be in their prime.

In Willis's case the illness began with primary progressive aphasia, the subtype that attacks language first. A person starts not finding the word. Then can't put a sentence together. Then doesn't understand what's being said to them. And all that time their memory works almost normally - unlike Alzheimer's, where recall goes first. Which is exactly why families spend so long thinking it's something else.

The symptoms are a list anyone who has cared for an older person will recognise with an uncomfortable feeling: difficulty speaking and understanding, major personality changes, impulsive or socially inappropriate behaviour, loss of empathy, an inability to plan and decide, misreading other people's emotions.

Read that list again. Not one of those symptoms looks like an illness at first glance. It looks like character. It looks like a man who has become rude, selfish, inattentive. That's why frontotemporal dementia destroys relationships first and bodies second - the partner gets angry, the children withdraw, friends call less often, and all of it happens before anyone even thinks of a neurologist.

Emma Heming Willis, his wife, described it in her book "The Unexpected Journey". Her sentence is the most accurate thing written about this illness: "I don't need Bruce to know I'm his wife, I just want to feel I have a connection with him. And I do."

There is no cure so far. Nothing halts the course of the disease. What is done is maintenance - speech therapy to hold on to the speech that's still there, neuropsychology, physiotherapy, occupational therapy, psychological support. Not to bring the person back, but to hold on for as long as possible.

And there's a second patient, rarely written about. The one doing the caring. Anxiety, depression, physical exhaustion, a feeling of isolation - those are the standard consequences for carers. Here that usually means one daughter or one wife, with no relief, no break and no one asking whether she needs help. The whole world writes about Bruce Willis. The question is who writes about her.