The eastern coral snake carries red, yellow and black rings in which red bands touch yellow ones, while the scarlet kingsnake and scarlet snake that mimic it have red touching black. Within the southeastern United States that colour order is a usable field discriminator, and the rhyme built on it is broadly sound there.
It is not a law of nature. The genus Micrurus contains dozens of species through Central and South America with banding patterns that do not follow the North American arrangement, including species where red meets black. Some coral snakes are not tricoloured at all. A visitor applying the rhyme in Costa Rica or Brazil has no protection from it.
Bites in the United States are uncommon because the snake is secretive, small-mouthed and reluctant, though its venom is a potent neurotoxin and envenomation is a medical emergency. Supply of North American coral snake antivenom has been intermittent for years, a practical problem quite separate from the biology.