Depression is a very difficult state of mind.  When an individual is in a low mood, it can affect almost every aspect of his or her life - behavior, feelings, thoughts, and even well-being.  Do you know anyone who battles with this?
I do actually, i know many who struggle with depression personally. It is a truly devastating mental illness. 
I agree.  It runs in my wife's family.  Obviously, things like the loss of a loved one can trigger it; but there are so many causes, including physical, hormonal, or mental issues that can trigger depression.  Have those friends of your been diagnosed with clinical depression?
They have yes. The causes however were due to hormonal imbalances and they are being treated with CBT and SSRI's.
always have symptoms present. Major depressive disorder can negatively affect a person's personal life, work life, or education, as well as sleeping, eating habits, and general health. Between 2–8% of adults with major depression die by suicide, and about 50% of people who die by suicide had depression or another mood disorder.
The cause is believed to be a combination of genetic, environmental, and psychological factors. Risk factors include a family history of the condition, major life changes, certain medications, chronic
Depression (mood)
Depression is a state of low mood and aversion to activity. It can affect a person's thoughts, behavior, motivation, feelings, and sense of well-being. It may feature sadness, difficulty in thinking and concentration and a significant increase/decrease in appetite and time spent sleeping, and people experiencing depression may have feelings of dejection, hopelessness and, sometimes, suicidal thoughts. It can either be short term or long term. Depressed mood is a symptom of some mood disorders such as