"Contagious" Depression?

Hello lovely beings,

I'm just looking for answers, am very confused.

They say depression is not contagious. However, how come it affects your closest ones so much?

I've been depressed for over 10 years and every time I've tried to tighten a bond with someone, be it friendly or romantic, I would always discourage people and put them off. Now I always try to "warn" people and tell them about my problems and that they should not take too much on their plate. I want to fix myself, but it's always small steps.

My question is, are relationships with a person with a mood disorder similar to e.g. codependency in alcohol-stricken relationships?

Does a friend/loved one of a depressed person become "ill" in a way as well?

What are the mechanisms behind it?