Friends sick at the same time every time

I have a close friend whom I care for and love as family. I've been noticing lately that when I feel sick so does he and vice versa. And when we are sick it's the same symptoms or the same pain. This has been happening for like 3 months now, and were either sick at the same time or one of us would feel sick within a short period if time after finding out the other is sick or we find out that were both sick at the same time. Sometimes we sit there and think what did we do that caused both of us to feel the same way. I'm curious as to why it happens.