An epidemic will continue if, on average, each infected person meets and infects one healthy person, known as a “susceptible.” If you get the flu and pass it along to one other and then he or she passes it along, etc, it will be a bad season. You might even catch it again when it comes back around in mutated fashion. But if the “bug” is not so strong and many non-susceptibles are in the population, so the average falls below one, the disease will taper out. The same with a joke or gossip at the office. If it’s funny or juicy and folks keep passing it along, averaging one susceptible (hasn’t heard it yet) per teller, it will spread. Some good jokes may come back around mutated, so as with the flu, you “catch it” (laugh at it) again. But, said Holland, since populations are not infinite and some people are “immune,” spreading is limited. Though this is what prevents people from getting rich using chain- letter schemes, it is also fortunately what kept the bubonic plague from wiping out all of Europe. Look for “Strange but True” every Saturday in the U section.165Let’s talk business.Catch up on the business news closest to you with our daily newsletter. Something went wrong. Please try again.subscribeCongratulations! You’re all set! Regarding that rainbow of rains around the world, what can color them red, yellow, brown, black, white, even gold? The red tints, suggestive of blood and dire warning, were noted by the Roman historian Plutarch, and by medieval bishop and historian Saint Gregory of Tours, who stated that one Easter Sunday the whole sky above the city of Soissons seemed to catch fire and in Paris “real blood rained from a cloud,” said Arizona State University climatologist Randy Cerveny in his book “Freaks of the Storm.” Charles Darwin himself noted hazy air while onboard the HMS Beagle and attributed it to falling fine dust the accepted explanation today for red rain, as huge reddish Saharan dust storms may be windswept across the Mediterranean and carried to Europe and Great Britain. “Sulfurous,” “diabolical” yellow rains as well go back thousands of years, now pegged to flower and tree pollen. Brown is usually linked to mud, as huge quantities of dust get upswept during duststorms or fires; black to volcanic particles or coal field soot; milky to thin clay particles from the Sahara. “There have even been gold rains.” Following massive dust storms in 1930, a Los Angeles assayer scooped some up and found gold and silver dust from desert regions hundreds of miles away. What do jokes, gossip, chain letters, AIDs and the flu have in common? All can spread like the plague, said Bart K. Holland in “What Are the Chances? Voodoo Deaths, Office Gossip & Other Adventures in Probability.” And disease is a fitting metaphor here, for the same math that governs the spread of germs steers the dissemination of the others. First comes the big “thrill,” but with repetition, tolerance sets in. Stopping doesn’t return you to where you were but rather triggers withdrawal symptoms malaise, depression, the blahs. The emotional fire in your heart has cooled to a glowing ember. What has fled, and what with luck will take its place? Passionate love has left you join the millions! “When you’re in love it’s the most glorious two-and-a-half days of your life,” jested comedian Richard Lewis. Actually, two years is more like it, said David G. Myers in “Social Psychology,” as by then spouses express affection only half as often as when they were newlyweds. About four years after marriage, the divorce rate peaks worldwide, said anthropologist Helen Fisher. It’s at this stage that the fires of addiction fade, just like to caffeine, alcohol, other drugs. “If a close relationship is to endure,” said Myers, “it will settle to a steadier but still warm afterglow that has been termed companionate love.’ ” This is a deep, affectionate attachment and just as real. But of course it doesn’t always develop. If passionate love often leads to children, this is obviously natural and adaptive for our species, just as it may be adaptive for the parents to experience some waning in their obsession with each other, said Myers. Later, as the nest empties, some of the old feeling may return, though many couples have parted ways long before.