The correct answer is mental illness.
Mental illnesses are among the most common health problems facing Americans, with 20 percent of adults having a diagnosable mental, behavioral, or emotional disorder. That's according to a new report from the Substance Abuse and Mental Health Services Administration.
That's 45.9 million people who have to deal with depression, anxiety, and other common disorders. But it doesn't include people with substance abuse problems, even though they are far more likely to have mental health issues, too.
Mental health problems are more common in young people than in the old. Almost one-third of teenagers and young adults ages 18 to 25 experienced a mental illness in 2010, compared to 14 percent in people age 50 and older.
Only about 40 percent of people with a mental health problem typically get treatment for it, although that number rises to 61 percent among people with a serious mental illness.