I do agree that attitude is contagious.
Because we people are like sponges, we can easily absorb or influence every persons around us especially those with whom we have close association.
There is a saying that " tell me who your friends are and i tell you who you are." It means that if your association or bunch of friends are good, then you are good, but if your friends are bad then you are bad.
We really can't tell that your gonna imitate their actions, but we already attached ourselves emotionally to them, they can have molding effect on us - for good or for bad.
So its really vital that we seek out association who will influence us for good.