On a foggy Saturday morning in September, 1972, Hans Kretschmer went surfing off Big Sur. About a hundred feet out, something bumped him. He thought that it might be one of his friends, or a seal, but when he looked over his shoulder he saw a huge head clamped onto his leg. He punched it. The ocean around him frothed and he became disoriented; he glimpsed a fin and a tail. Kretschmer began swimming. When he made it back to the beach, there were three bloody gashes in his thigh. The creature was gone. “I described it being shiny, black, and white to the ambulance drivers,” Kretschmer told me.

If the people who treated Kretschmer’s injuries had checked the International Classification of Diseases, a system of codes that is used in medical record-keeping, they would have failed to find a precise descriptor for what likely happened to him. But the latest edition of the I.C.D., which was introduced in the United States in October, has just the one: W56.21, or “bitten by orca.” Boasting around a hundred and forty thousand codes—up from about eighteen thousand in the previous version—the I.C.D.-10 is simultaneously actuarial and suggestive. A better title for parts of it, to borrow from the author Daniel Handler, might be “A Series of Unfortunate Events.” It covers traumas and health conditions caused by sea lions, geese, ducks, turkeys, and macaws, with separate codes for being “struck by” or “pecked by” a chicken. (There is also the mysterious “contact with nonvenomous frogs.”) Various human factors are codified, too, including “problems in relationship with in-laws,” “sibling rivalry,” “rough housing and horseplay,” and “hostility towards and scapegoating of child.”

The revision has inspired dark conjecture. “If it was a droid, I.C.D.-10 would serve Darth Vader,” Robert M. Wah, the then president of the American Medical Association, said last year. Its implementation will require software upgrades and personnel-training sessions, both of which are potentially expensive, and there are fears that insurers could begin denying claims more frequently, on the grounds that a new code was misapplied. (So far, the Centers for Medicare and Medicaid Services reports rejection rates of only a tenth of one per cent due to bad I.C.D.-10 codes, although it is too soon to draw firm conclusions.) Even professional coders who are otherwise in favor of the revision are puzzled by particular entries. For Rhonda Buckholtz, who specializes in I.C.D. training, these include “exposure to ignition of plastic jewelry” and injuries caused while “knitting and crocheting.” (Asked about “drowning and submersion due to falling or jumping from burning water-skis,” Buckholtz replied, “You can’t stop, drop, and roll in the ocean.”)

The codes have occasionally been a politically partisan issue. “I’ve asked physicians all over the country: Have you ever seen an injury from a macaw?,” Rand Paul, the Kentucky senator and current Presidential candidate, said during a speech at an Iowa Republican Party dinner, in 2013. He was implying a link between Obamacare and classificatory overexuberance, but the I.C.D.-10 is not, in fact, the product of Washington bureaucracy run amok. Its unlikely starting point, according to a history published by the National Center for Health Statistics, was an event in 1851—the Great Exhibition of the Works of Industry of All Nations, which was held at London’s Crystal Palace. In addition to putting various cultural oddities on display (the German Customs Union sent in a tableau of stuffed kittens taking tea), the Great Exhibition spurred an interest in statistical comparisons of industrial products. A few years later, at a global statistical congress in Brussels, it was proposed that causes of death be classified and contrasted. One of the resulting lists, a forerunner of the I.C.D., had categories such as “deaths from congenital debility, malformations or monstrosity” and “deaths from ill-defined diseases.”

The modern I.C.D. is issued by the World Health Organization, and the United States and other nations adapt it to their own needs, with physicians, pharmaceutical companies, and others suggesting amendments. In the case of the injury and illness codes, this involves an exhaustive, finicky process that is shepherded by the Centers for Disease Control and Prevention. The documents from those meetings read like an abbreviated history of America in the twenty-first century. Extra anthrax codes were proposed in 2001, codes for gluten sensitivity in 2013. In 2008, meanwhile, the Department of Defense requested codes for dozens of activities—tuba playing, synchronized swimming, rollercoaster riding, tap dancing, sheep shearing—as a way to track the health of the military population. Not all of these suggestions make the cut. “A question was raised on what codes would apply if a mother was given the wrong baby to breast feed,” the summary of one meeting read. “It was stated that would be outside of the scope of ICD for coding.”

The United States is, by the federal government’s own admission, the last major industrialized nation to adopt the I.C.D.-10. Still, its expansiveness does not trouble everyone. “There are thousands of words in the dictionary,” Donna Pickett, a C.D.C. classifications administrator, told me. “No one uses all of them at the same time—some are archaic and may never be used. But that doesn’t necessarily mean you take the words out.” Proponents note that many sections of the previous edition were lacking. Ebola didn’t have a code, and new advances, such as some types of laparoscopic surgery, had to be shoehorned in under old categories. The I.C.D.-10 allows for greater precision, which is good news for epidemiologists. Geoffrey C. Bowker, an informatics researcher at the University of California, Irvine, praises it for the same reason. What if you wanted to research hazards in a specific sort of gathering place? “We want to know what happens in opera houses,” Bowker said. “We want to know if there’s a particular kind of danger that’s associated with attending the opera, which I can’t particularly imagine, apart from boredom.”

Perhaps the creators of the I.C.D.-10 ought to be commended for their scrupulous parsing of all that can befall the human body. Instead of bureaucratic and nitpicking, the system could be viewed as uncompromising and ambitious, a bid to give form to chaos. Indeed, it is possible to become so immersed in the codes that one begins to see the world in their terms. Jaci Johnson Kipreos, a consultant in San Diego, has applied them to famous films. For “Titanic,” she suggests “exposure to excessive natural cold” and “inappropriate (excessive) parental pressure”; for “Ben-Hur,” “non-running track and field events.” Similarly, Rhonda Buckholtz said that she casually classifies things all the time. Once, on a visit to National Harbor, Maryland, with her husband, she stopped to photograph a statue of a Renaissance-era dwarf, naked, riding a reptile. It was a replica of a figure in Florence’s Boboli Gardens. “There’s a code for that,” Buckholtz said. She was thinking of “other contact with turtle.”