the ability to meet the demands of everyday life without becoming exhausted; ability of the heart, lungs, muscles, and joints to perform at their greatest capacity

depression is an ongoing sadness\lack of motivation. you feel and feel like its never going to get better. you're so sad to the point you think of death.

anxiety is an ongoing feeling of paranoia\ feeling like everyone is watching you and making fun of you. you get nervous easily and feel like the room is spinning.  

yes, they cause death every year. its a long, exhausting battle. only few make it through as sad as it sounds.

explanation:

wellness is the state of being in good health, especially as an actively pursued goal.