Laughter is contagious, yes

And now, we may know at least part of the reason why

It appears to stimulate reaction in another person’s premotor cortex. Although the linked study doesn’t mention them by name, I’m guessing that mirror neurons are somehow involved.

“We’ve known for some time now that when we are talking to someone, we often mirror their behaviour, copying the words they use and mimicking their gestures. Now we’ve shown that the same appears to apply to laughter, too — at least at the level of the brain,” Sophie Scott said.

This would appear to confirm that idea.