We may worry about the threat of terrorists, but there is a greater threat close to home – right inside our heads.

In a heartrending article entitled, “The Terrorist inside My Husband’s Brain”, Susan Williams describes the mental demons tormenting the mind of her spouse Robin, the renowned actor.

“Robin was losing his mind and he was aware of it. Can you imagine the pain he felt as he experienced himself disintegrating?” she wrote in the journal of the American Academy of Neurology last week.

Robin Williams was actually suffering from an unusually extreme case of Lewy Body dementia, which caused paranoia, anxiety attacks, insomnia, gut problems and memory loss in the “tragic and heartbreaking” months before his death by suicide in August 2014.

In early April, during the filming of Night At The Museum 3, he had a panic attack and had trouble remembering even one line. His wife had trouble calming down his fears, and an antipsychotic drug even made things worse, as it was not the right drug for his condition. He had wrongly been diagnosed for Parkinson’s.

“My husband was trapped in the twisted architecture of his neurons and no matter what I did I could not pull him out,” Susan wrote.

Perhaps the saddest part of this story is that it is actually a common one, yet due to the shame and stigma, many sufferers of mental illnesses stay in the shadows.

If you have four or more people in your family, or a group of friends, the chances are, one member will suffer from a mental disorder at some point, since on average, 20 to 25% of people are affected in a lifetime, says the World Health Organization (WHO).

WHO also estimates that at any one time, one in four families has at least one member suffering – invariably silently – from a mental disorder.

Depression, or the “black dog”, as Winston Churchill called it, affects some 350 million people. By 2030, some 76 million people will have dementia, including Alzheimer’s.

Suicide, which is associated with mental disorders, is a leading cause of death of young adults worldwide. It now accounts for more deaths in India among women (aged 15 to 49 years) than complications from childbirth and pregnancy.

It’s an enormous, unspoken burden. And it costs economies. WHO has calculated the loss of productivity due to mental disorders amounts to more than 10 billion days of lost work annually – the equivalent of US$1tril (RM4.16tril) per year. Mental illness is also an underlying driver behind poor diet, and alcohol and drug abuse. Thus by treating mental disorders, we also battle drug addiction.

Yet many developing countries spend very little on treating and preventing mental illnesses – often less than RM8 per person, WHO estimates. Caregivers are left with the burden of coping.

In Malaysia, the recent National Health and Morbidity Survey found nearly 30% of Malaysians suffer mental health problems. Most affected were teenagers and people from low-income families. The poor, typically, suffer more from mental disorders, particularly among the unemployed, or people living with debts or in poor housing.

Because of the stigma involved, many sufferers do not acknowledge their mental problems. Many, also, get little support from family and friends. Even fewer ever receive treatment.

Yet many mental disorders can be treated. There is psychological care, as well as antidepressants and antipsychotic drugs. Other methods can be offered within the community to provide early intervention such as a “life skills” programme in schools, which can also help improve learning outcomes, or a “stress reduction” workshop at work.

Worldwide there is a shift away from shutting people away in institutions, which all too often are scenes of human rights abuses.

There is another way to both treat and prevent depression: sports. Many, many studies worldwide show that regular physical activity can reduce or even prevent mild to moderate depression. Imagine if we invest big time in sports, providing sport centres to the young across the nation. How much would that contribute on so many levels to the nation?

In the long run, there is another area we need to address: housing. A huge number of people live in poor housing; at worst, as squatters.

Tomorrow is World Mental Health Day. Perhaps the first step to improving mental health is talking about it. And listening, too, to those who suffer. Sometimes even the simplest of actions can go a long way.

Mangai Balasegaram writes mostly on health, but also delves into anything on being human. She has worked with international public health bodies and has a Masters in public health.