What do J.K. RowlingMegan Fox, David Beckham and Jim Carrey have in common? All of them have at some point suffered from a form of mental illness or a mental health problem, which can range from depression and obsessive-compulsive disorder to anxiety and bipolar disorder. These celebrities and others have begun to dismantle the stigma associated with having or talking about mental illness, but the world still has a ways to go. That's why World Mental Health Day, marked Oct. 10 every year, exists: to spark conversations and learning about the conditions that affect some 450 million people all over the world. 

Mental health is a broad term encompassing a person's "emotional, psychological and social well-being," according to MentalHealth.gov. That means a person's mental health affects not just how they think and feel, but also how they act, react and make decisions. A plethora of factors influence mental health, from genes to environment to life experiences, and symptoms can range from "eating or sleeping too much or too little" to "feeling unusually confused, forgetful, on edge," to "hearing voices or believing things that are not true."

If those descriptions and definitions seem vague or exceedingly diverse, it is symptom of the fact that mental illness and mental health are both difficult to talk about and poorly understood, advocates say. As a result, people with mental illnesses -- 1 out of 5 adults in the U.S. in any given year -- are often stigmatized or do not get treatment, all because of a pervasive lack of understanding of what it means to have a mental illness.

Consider, for instance, the way words like "crazy" or "insane" are tossed around in casual conversations to describe silly behavior or decisions that hardly match the medical definitions of the terms. In the wake of mass shootings, a suspect's history of mental illness is often one of the first details people want to know about, a misguided focus that comedian John Oliver has masterfully criticized:

Jokes aside, almost everyone is affected, directly or indirectly by mental illnesses, which account for 13 percent of the global disease burden, according to the International Medical Corps. People with mental health disorders contend with higher rates of unemployment, for instance, while families of people with mental illness may have to assume caretaking responsibilities.

Yet people with mental health conditions "are not only discriminated against, stigmatized and marginalized but are also subject to emotional and physical abuse in both mental health facilities and the community," the World Health Organization said in advance of World Mental Health Day. People with mental health conditions often suffer twice over, from both the condition itself and the resulting stigma or lack of support from their communities.

Two glaring indicators of the lack of support for people with mental illness are the facts that 26 percent of homeless adults living in shelters have a serious mental illness and that 24 percent of prisoners in the U.S. have a history of mental illness, according to the National Alliance on Mental Illness, a Washington, D.C., advocacy group.

In the United States, nearly 1 in 25 adults has a serious mental illness, and 2.4 million adults live with schizophrenia, 6.1 million with bipolar disorder, 16 million with depression and 42 million with anxiety disorders, the alliance has said.

World Mental Health Day is supposed to be an opportunity to both challenge stigma and educate people about mental health. It has "the overall objective of raising awareness of mental health issues around the world and mobilizing efforts in support of mental health," according to the World Health Organization.