On Thursday, U.S. hospitals, doctors and other care providers have to start using internationally developed standards called ICD-10 codes to bill government programs and private insurers in the nation’s $2.9 trillion-a-year health-care system. The codes cover everything from parrot bites to getting sucked into a jet engine.

Doctors have already begun mocking the diagnostic list, picking out the most absurd and arcane, such as Z63.1, “Problems in relationship with in-laws,” or V91.07XA, “Burn due to water skis on fire.” Yet for health-care providers who have payments at stake, the complex change is no joke.

Health insurers and government programs stopped accepting the old set of codes, called ICD-9, in the early morning hours Thursday, completing the switch. In a study released last month, the Government Accountability Office, the investigative arm of Congress, said “little is known” about how much it will cost the health-care system to make the transition. HCA Holdings Inc., the Nashville, Tennessee-based hospital chain, said 2015 costs for the transition to the new codes will be about $30 million.