Nearly one in five American adults -- 43.8 million people -- experienced some type of diagnosable mental illness last year. The latest findings, from the Substance Abuse and Mental Health Administration's (SAMHSA) 2013 National Survey on Drug Use and Health, show just how widespread such issues really are.

More than 15 million people in the U.S. over the age of 18 suffered from an episode of major depression in 2013. More than nine million reported having serious thoughts of suicide, while 1.3 million actually attempted suicide. SAMHSA said these figures were comparable to the numbers for 2012.

Major depression was also a problem for about one in 10 adolescents ages 12 to 17, or about 2.6 million teens. Only about 38 percent of them received treatment.

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Even though mental illness is now spoken about more openly and recognized as a common, often treatable condition, many of those who suffered from it failed to get treatment.

About 68 percent of adults with serious mental illness got some type of mental health care last year, as did just under half of those with "moderate" illness.

"It is a serious issue that millions of Americans are needlessly affected by mental illness when they can get effective treatment to restore their well-being," SAMHSA Administrator Pamela S. Hyde said in a statement. "Now more than ever, people can get the help they need to recover from mental disorders and live full, active lives - they just need to take the first step and seek help."

According to the survey, the most common reasons people who needed mental health treatment did not get it was that they thought they couldn't afford it. Others said they thought they could handle the problem on their own, or didn't know where to go for help.

SAMHSA has an online guide to help anyone who needs it find treatment programs near them. It also runs a 24-hour, toll-free information line at 1-800-662-HELP (4357).