As someone who has suffered immensely from anxiety, I know this to be wholeheartedly true.
Humans are a social species. Instinctively, we thrive in communities. We have a very basic desire to belong, to be a part of a community or social network.
At the same time, we are a reflection of those around us. We are very strongly influenced by our peers, family, and friends.
Happiness is contagious, but so is depression. Mental illness presents a burden not only to the sufferer, but also to those close by. It is a dilemma. I love that research is so dedicated to exploring this! It is much-needed.