Depression is when a person experiences low mood and an aversion to activities that affects their thoughts or behaviors. It can be very limiting. 
I have experienced it in the past. It's very hard to live with. How many people in the US suffer from it?
I'm not sure how many people suffer but I know it is a major crisis. A depressed mood can be a symptom of a disorder such as major depressive disorder or dysthymia. 
What are some other symptoms? I suffered from it for years. I was put on Zoloft.
Symptoms can include sadness, anxiety, a feeling of emptiness, worthlessness and a sense of hopelessness.  
I had a medium level case of it. What are some of the treatments used?
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
ray detector technologies.
DQE especially affects one's ability to view small, low-contrast objects. In fact, in many imaging situations, it's more important to detecting small objects than is limiting spatial resolution (LSR) - the parameter traditionally used to determine how small an object one can visualize. Even if a digital system has very high LSR, it can't take advantage of the resolution if it has low DQE, which prevents the detection of very small objects.
A study comparing film/screen and