The exact cause of depression isn't clear. No one is sure why some people get depressed
and others don't. Sometimes depression seems to happen because of a stressful event.
Sometimes it seems to happen for no reason at all.
Genes could play a role. People whose blood relatives have had depression are more
likely to have it, too. However, not everyone who has a relative with depression
is going to develop it.
Today, it's widely understood that depression is a serious medical condition. Scientists
believe that it could be linked with an imbalance of a chemical in the brain called
serotonin. If this imbalance happens, it can affect the way people feel.