Mental illness isn't contagious; there can be both genetic and environmental influences to them though, and researchers still don't know/agree about them fully.

That said - depressions fluctuate. They can be suicidal, or lethargic, or angry, or mild, and anything else really. And someone who has depression isn't stuck with it 100% of the time - episodes dissipiate over time (how long of a time depends) and even within the episode the intensity can increase and decrease.

What you have outlined in your brief post is that you went into an environmental depression based upon family issues. 6 months after breaking up with your boyfriend and you were able to feel like your normal self again. That's 6 months of processing whatever was going on in your family. It doesn't sound like he had any impact on it, as you haven't mentioned anything he might have been doing that would contribute to feeling poorly.

If you're feeling a little bit low right now, that doesn't mean it's depression or that a depression is returning. It could just be feeling of guilt over breaking up, feelings of being replaced as he had a new relationship for a while, worries over having a depression come back if you try things with him again and then breaking up again.... or all sorts of other things.