Adopted from NAMI (National Alliance on Mental Illness):

A mental illness is a condition that affects a person's thinking, feeling, behavior or mood. These conditions deeply impact day-to-day living and may also affect the ability to relate to others. If you have — or think you might have — a mental illness, the first thing you must know is that you are not alone. Mental health conditions are far more common than you think, mainly because people don’t like to, or are scared to, talk about them. 

• 1 in 5 U.S. adults experience mental illness each year
• 1 in 6 U.S. youth aged 6-17 experience a mental health disorder each year
• 50% of all lifetime mental illness begins by age 14, and 75% by age 24