But seriously folks, laughter isn’t always funny

Tumbling puppies. The latest YouTube making the rounds. Will Ferrell on the big screen. All can provoke the flexing of facial muscles and respiratory contractions that amount to a laugh. Genuine laughter (and possibly even forced laughter) may be, if not the best medicine, then a pretty good one.

But laughter sometimes has a more somber side.

Back in the ‘60s, now-emeritus Stanford psychiatry professor Dr. William Fry embarked on a research agenda that would ultimately help demonstrate that laughing was good for human health. Among other things, Fry showed that a minute of hearty laughter elevated the heart rate as much as 10 minutes on a rowing machine did.

Norman Cousins, a magazine editor who later joined the faculty of the medical school at UCLA, popularized the healing effects of laughter in his 1979 book, “Anatomy of an Illness.” Cousins had been diagnosed with a rheumatic disease called ankylosing spondylitis -- and treated himself by taking daily doses of vitamin C and dosing himself with hearty helpings of chuckles (he was especially fond of watching the Marx Brothers).

Cousins went on to become an eloquent advocate for holistic medicine in general and the therapeutic effects of laughter in particular. But although he claimed that laughter cured him, not all observers were convinced it was the giggles that did the trick.

Laughter came under increasing clinical scrutiny in the years that followed. Studies in the 1980s and 1990s showed that chortles and whoops can improve mood, mitigate stress and lessen the effects of depression and anxiety. Other studies suggested why this was. Chuckling, it turns out, elevates levels of endorphins (painkilling chemicals produced by the brain). Following a hearty laugh, blood pressure and heart rate drop, and the body relaxes.

In recent years, Loma Linda University researchers have found that laughter increases the production of some immune cells while spurring others into action; it also appears to reduce levels of the immune-depressing stress hormone cortisol.

But laughter has a dark side too. Laughing allegedly brought on the stroke that killed British novelist Anthony Trollope and caused temporary blindness in a Massachusetts man cracking up at an episode of “Seinfeld” -- mysterious occurrences that still have scientists perplexed.

Inappropriate laughter is also a hallmark of a handful of rare medical conditions, an indication that something has gone awry somewhere in the brain.

The lobotomies commonly performed in mental hospitals in the 1940s and 1950s occasionally resulted in a new problem for the patients: fits of uncontrollable and involuntary laughter, brought on by damage to the brain’s frontal lobe, the seat of voluntary movement and emotion.

A rare developmental disorder known as Angelman Syndrome is accompanied by seizures, jerky movements and paroxysms of inappropriate laughter.

A type of epilepsy, known as gelastic epilepsy, is also characterized by fits of out-of-place laughter; “gelos” is the Greek word for laughter.

Kuru, a now very rare degenerative brain disease affecting the natives of New Guinea, and a related disease called Creutzfeldt-Jakob disease, are both accompanied by seemingly unprovoked spells of laughter. Both diseases involve such extensive brain degeneration (caused by infectious proteins known as prions) that scientists are still unsure how, precisely, they cause such pathological laughter.

Diseases aren’t the only cause of out-of-context laughter: Toxic substances can bring it on too. Manganese poisoning once caused clumsiness and chuckling in workers who mined the mineral for a living. A deadly herb native to the island of Sardinia causes the same strange laughter before resulting in the death of its still-smiling victims.

And then, of course, there are the less-toxic, laugh-inducing gases nitrous oxide and ether. Nitrous oxide, commonly known as laughing gas, first became a popular recreational drug back in the 19th century. In 1844, an astute dentist decided the feel-good gas might have a place in his clinic too. To test his hypothesis, he had one of his own wisdom teeth extracted while intoxicated. Painless dentistry for all of his patients (and also others) ensued.

The conditions that cause inappropriate laughter aren’t communicable, but laughter often is. Humans laugh when they hear laughter (recall the sitcom laugh track), and laughter spreads easiest in large groups of people.

Sometimes it spreads like wildfire. In January 1962, three teenage girls in a school in what is now Tanzania erupted into uncontrollable hysterics. Soon more than half the girls in the school were convulsing with giggles, and the staid teachers were forced to suspend all classes. The girls took their contagious laughter home with them, ultimately infecting close to a thousand people with guffaws to the point of breathlessness and tears.

Something similar happens in some Pentecostal churches in the U.S. today, in which pastors infect their congregations with fast-spreading, contagious, whole-body laughter. Not only is laughter likely good for the body; some, it seems, are convinced it’s good for the soul.