It's estimated that one million people in the UK are living with the long-term effects of a brain injury.
In his new documentary 'A Different Brain' which aired on Sunday 15 May, Louis Theroux spent time with just a handful of these people in order to understand the endless issues they face during recovery.
He met with people who had brain injuries so severe they'd forever rely on round-the-clock care, as well as those who had experienced injuries and were trying - and often struggling - to gain their independence again.
While many of the stories are heartbreaking, they showed that even in the bleakest of situations, love from family members and friends prevails.
In the new documentary, Louis met with staff and patients of the Brain Injury Rehabilitation Trust, one of the UK’s largest providers of neuro-behavioural rehabilitation.
He wanted to gain a better understanding of how individuals and their families come to terms with such a life-changing condition.
While brain injury is very much a 'hidden disability', those affected by it are left with the huge challenge of reconstructing who they are.
From relearning how to walk, talk and eat, to redeveloping personality and behavioural traits, it can be a very confusing and difficult existence for patients.
And it can be even harder for family members, who are caught between grieving for the loved one they’ve lost and learning to love the person they are now.
One of the patients Louis meets is Earl, who was involved in a fatal car crash two years ago and sustained a head injury. There were two passengers also involved in the accident – Earl's friend, who lost his life, and his ex-girlfriend.
"Earl pulled in front of an ambulance and clipped the curb," his mum Patricia told Louis.
"[He] lost control of the car, the car went across the road and he ended up going down on the tram lines and it wrapped around the tram line pole.
"I’m made to believe he was going about 50mph," she said. "He’d only been driving for about two and a half weeks."
The crash not only left Earl physically injured, but with a terrible brain injury too.
"I had to learn to walk again, talk again, eat again,” he said.
Following the accident, Earl was charged with death by dangerous driving but was unfit to stand trial and was given a supervision order overseen by the brain injury unit.
For Earl's family, one noticeable effect of his brain injury was his complete change in personality. Not only did he become noticeably angrier and aggressive, but his tastes also changed.
Before the accident Earl loved cooking, but now he is unable to do it. His taste in clothing has also changed, as well as his daily habits and quirks.
"Earl just wants all expensive stuff now, before he wasn’t bothered," explained his mum. "But now he just wants all name-brand clothing."
Earl's brain injury has resulted in him developing unusual habits too, such as cleaning his teeth numerous times throughout the day.
"At home, he has to have two different toothpastes, if not three. He does things in three and fives for some reason," added Patricia.
When Earl returned home following the accident, he would throw things at his mum and be verbally abusive to her.
"Earl has come back a completely different person," she said with tears in her eyes.
"I’ve got a different son. He looks like my son, that’s Earl, but he’s got a different soul inside him.
"The only thing that’s the same about Earl is his looks."
And even his looks aren't entirely the same, she added, as he now has a "dead look" in his eyes.
Despite her son's abusive behaviour towards her and complete change in the way he acts, Patricia insists her feelings towards him haven't changed.
"I love him just the same. If not even more," she told Louis, while they stood alone in her kitchen. "He’s a different person, but he’s still my son.
"I don’t like his behaviour. I don’t like his actions at times. But I love my son and I always will, regardless."
Even to this day, Patricia believes she is partly to blame for his injury.
She told Louis that she believed she could’ve prevented the accident by not buying him a car. "It’s that simple," she added, with tears in her eyes.
So what does the future hold for Patricia and her son?
"Just got to keep strong and give him a strong foundation to keep learning and hopefully he’ll get it," she said.
"If he doesn’t get it, then I’ll just continue to do what I’m doing until my days are done. And then his sister will look after him, because we’ve got no choice."
While relationship will clearly never be the same again, Patricia's love for her son is stronger than ever. And, to a degree, Earl's love for his mum is too.
The emotional show also follows the story of Amanda, a former vet who fell off a horse, as she tries to rebuild relationships with her husband and children, as well as Dan, who wants desperately to gain independence again, and Natalie, who has been in a brain injury unit for 15 years.