Depression is often called the common cold of mental health. While that probably is a good analogy for the frequency of depression, it is not a good analogy for its potential severity. Everyone goes through times when they are down and depressed, but major depression can suck the life right out of a person, leading to serious medical issues and thoughts of suicide. Depression can be a killer and should be taken very seriously. Below are some good resources for you to learn more about depression.
Depression: What you need to know
5 Action Steps for Helping Someone in Emotional Pain