The introduction is a great attention-grabber, although consider adding a few things. For example, when you explain the symptoms of depression, mention they're in the DSM-IV-TR, although you don't need to explain the entire diagnostic criteria. Also, you mentioned depression is prevalent but it would be more informative if you found results from a survey (the larger the sample size the better, so look for national ones). A good source would be the National Depression Survey (note: pdf document). These are just to add a bit more power to your speech and make it informative.