Editor's note: Across Kenya, millions of mentally disabled people are hidden away: locked up and forgotten, often by families who can't get them proper treatment. Watch "World's Untold Stories" on Saturday and Sunday.

Nairobi, Kenya (CNN) -- The tin shack looks like any other in a patch of small plots on the dusty outskirts of Nairobi. It's the haunting sound that grabs you, the awful moaning and cries coming from within.

It's Thomas Matoke's home. But it's more like a cell. Matoke, 33, is tied to a steel bedframe with a piece of blue rope. He's surrounded by pools of his urine, his mattress soiled and ripped to shreds.

His moans are interrupted when he chews his hand or the bedframe. He can't speak to tell his mother what he wants or feels. He's alone in his world of screams and agony.

He's been like this for 30 years.

Matoke got ill when he was a toddler and lost much of his high-level functioning. So his mother ties him up to prevent him from running away or hurting himself.

Countless trips to doctors and hospitals haven't helped him. And poverty means there isn't much medical help his family can afford.

"His siblings ask whether we wronged God, because we are really suffering," said his mom, Milkah Moraa. "I can't even hang his clothes outside because of the stink. The neighbors complain."

Shunned by the community, Moraa does what little she can to ease his agony. Her life is consumed by trying to take care of her sick son.

But Matoke is not alone.

There are an estimated 3 million, mostly poor, Kenyans living with intellectual and mental disabilities, according to NGO and United Nations figures.

As part of a special investigation, CNN found that families are struggling to cope with their loved ones, receiving little help from the state and facing massive stigma from society.

CNN's team filmed families locking up their loved ones, children discarded by institutions, cases of suspected sexual abuse. Kenya faces an epidemic of neglect.

"It is such a huge problem," said Edah Maina, head of the Kenya Society for the Mentally Handicapped. "If somebody would understand the extent it is huge, then I think someone can begin to act."

But often, Maina and her charity are the only ones acting. Scores of cases of neglect and abuse flood their office every day: autistic children chained in chicken coops, epileptic adults sealed in filthy shacks, daughters raped by their fathers. They are overwhelmed.

Dr. Frank Njenga, president of the African Association of Psychiatrists and a leading expert in the field, believes the scale is "catastrophic."

"We as a people have perfected the system of hiding our friends, relatives and other loved ones who have intellectual disability away from sight," Njenga said. "Out of sight, out of mind, no funding, neglected completely."

He says that the greatest neglect comes from the Kenyan government.

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The Kenyan government spends less than 1% of its health budget on mental health, though its own figures show that one-quarter of all patients going to hospitals or clinics complain of mental health issues.

And the Health and Medical Services ministries have been plagued by a series of corruption scandals in recent years.

More than $3 billion in public money was stolen in 2009, according to the Kenyan Ministry of Finance. This could have funded the entire ministry responsible for mental health -- for 10 years.

The minister of medical services, Anyang Nyong'o, says mental health is a high priority, but it needs more funding from his central government.

"It is definitely starved of resources, and that is not because we want to intentionally starve mental health; that is because the resource base as we have for running health services is very narrow," he said.

"The policy is very clear," Njenga said. "Mental health services are a priority in this country. The practice is also clear. They are not."

Whatever the cause, it is ordinary Kenyan families who suffer. And often, it's mothers who toil alone. Thomas Matoke's father has been absent for most of the past 30 years. Moraa says Matoke's condition pushed him out.

"For how long will I carry this burden?" she asked. "Since I got married, I have not had joy the way other people have joy. I have tried to encourage myself and think 'God help me, because I have carried this burden for a long time.' "

They have been chased away from village after village by angry, fearful neighbors. And soon, she fears, they will have to move away from this place as well.

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When the weather is good, she takes Thomas from his makeshift cell and ties him to an acacia tree outside. If she lets him go, he runs off. What she most wants is a place where he can get proper care.

But she says there is little chance of that happening.