Mother laughing with daughter.

It may look like fun, but uncontrollable laughter can be a symptom of something far more serious.

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Normally, we laugh because something's funny, but sometimes laughter can be something more serious -- a medical symptom. This sounds a little strange -- what do laughter and sickness have to do with each other? When we laugh, it's usually a sign of happiness. According to the science of happiness, intentional laughter can even uplift our mood and make us happy [source: Lamber]. But every now and then, you'll find yourself waiting in line at the bank or at the grocery store when someone just laughs suddenly and wildly for no apparent reason. He or she may have a tic and may make twitching motions, or may seem slightly disoriented. The person may be laughing and crying at the same time in a way that seems childish or violent.

When we laugh unintentionally and repeatedly, we may be experiencing pathological laughter. That's a sign of an underlying disease or medical condition that usually affects the nervous system, and researchers are still learning more about this phenomenon. (Pathological laughter usually isn't associated with humor, mirth or any other expression of happiness.)

As you probably know, our brain is the like the cockpit of our nervous system. It sends signals that control involuntary actions like breathing as well as voluntary actions like walking or laughing. When those signals go awry, due to conditions like a chemical imbalance, an abnormal growth in the brain or even a birth defect, they can cause bouts of odd laughter.

Let's learn more about these diseases and medical conditions that can cause the kind of laughter that doesn't make you smile.