The prevalence of anxiety and depression (A&D) is increasing worldwide. The impact of this illness — the costs its treatment invokes, as well as its increased prevalence — has invited comparisons with leading chronic diseases such as hypertension, diabetes, cancer and coronary artery disease.

So far that has not meant better, more accessible treatment for sufferers, who have too often remained in the shadows of healthcare. But awareness is growing.

One recent study, from Moody's Analytics, groups A&D with ailments like cancer and substance use disorder, whose impacts better correlate with family history and lifestyle than some other major, potentially fatal diseases. According to the report — prepared and released on behalf of the Blue Cross Blue Shield Association — the adverse health impacts of specific diseases can be linked in varying degrees to social determinants.

The impact of certain major chronic illnesses, such as hypertension, diabetes and coronary artery disease, correlate with social determinants of health, such as socioeconomic level, level of education, community health behaviors and local health system traits.

However, other major illnesses, such as cancer, substance use disorder and A&D, correlate less with social determinants. The impact of these illnesses is more influenced by individual factors such as family health history and personal lifestyle choices.

The grouping of A&D with those other serious illnesses is a sign that mental illnesses — and A&D in particular — are being taken more seriously than ever. It's also another validation of the role of mental healthcare in maintaining overall public health.

Another study by the World Health Organization (WHO) roughly estimates, for the first time, both the health and economic benefits (in particular, the ability of a treated patient to work) of investing in treatment for A&D. The WHO pointed to a fourfold return on such investment, considering that A&D disorders cost the global economy $1 trillion per year.

The prevalence of many other common mental disorders is on the rise along with A&D, whose sufferers now number well over 600 million or nearly 10 percent of the population. In perspective, mental disorders' share of the global nonfatal disease burden is 30 percent. During emergencies, disasters and ongoing crises, it is estimated that nearly 20 percent of a given population may be affected.

Calculating future treatment costs and health outcomes for 36 low-, middle- and high-income countries for the period 2016-2030, the WHO found that reasonably enhanced treatment — in the form of further psychosocial counseling and antidepressant medication — would cost $147 billion.

Yet even a 5 percent improvement in labor force participation-productivity in those countries would be worth nearly $400 billion. Workers' improved health would add an additional $310 billion in returns. Thus, the WHO concludes that the benefits of reasonably improved treatment for A&D far outweigh the costs.

Considering these factors, the WHO found current investments in mental health services in most of the countries surveyed to be falling far below what they should be. Governments spend an average of only about 3 percent of their health budgets on mental health.

The fact remains that A&D can be considered the leading cause of disability and lost productivity worldwide, according to a Stanford-led study. And yet smaller and smaller proportions of those sufferers receive treatment — likely because the total number of sufferers has grown.

Among the principal barriers to more pervasive treatment? It is not just a lack of healthcare coverage for mental health treatment that is an obstacle. A sufferer's condition itself may hamper his or her ability to seek, much less to receive, treatment.

That is among the findings of a study from the NYU-Langone Medical Center's department of medicine in New York City (which included national health data from a survey conducted by the U.S. Centers for Disease Control & Prevention of more than 35,000 households).

In an eight-year period spanning this decade and last, researchers found that people with serious mental illness had less access to healthcare services, compared to those without mental illness. This finding was due to various reasons, including the fact that nearly 1 in 10 distressed citizens in 2014 did not have health insurance that would cover visits with a psychiatrist or mental health counselor.

Still, as the recognition of A&D's prevalence is growing, so is the awareness that serious mental illnesses — in particular, diagnosable conditions like A&D — can greatly reduce, even destroy, sufferers' ability to function, compromising both the quality and the length of their lives.