Blood flukes are parasitic flatworms. They get their start living in snails, which shed the parasites into the surrounding water. If you go wading into a blood fluke-infested pond, the missile-shaped flukes will sniff their way to your skin and drill in. Once they reach a blood vessel they surf the sanguine tide until they reach your intestines. They take up residence in the blood vessels there, producing eggs that they nudge into the intestinal walls. The eggs get washed out of the body with their host’s stool, perhaps to infect a freshwater snail. Sometimes, however, the eggs get swept off in the wrong direction and wind up in the liver, where they cause chronic inflammation.

Getting blood flukes (the disease is known as schistosomiasis or bilharzia) is, sadly, nothing special. Two hundred million people suffer from infection with Schistosoma mansoni or a related species of blood fluke,Schistosoma haematobium. But the case of the man at the Royal Perth Hospital was singular in one respect. In order to get infected with blood flukes, you have to go to the places where its snail hosts live. And Australia is not one of those places. The man had, in fact, traveled to East Africa, where the blood flukes are common. But he hadn’t been there in 31 years.

This was remarkable, and more remarkable than you might think. It didn’t mean that the blood flukes had taken up residence in the man’s body and had produced 31 years of new generations. Remember, the eggs cannot develop inside a human body. They have to reach fresh water to hatch, and if they can’t find a snail to invade, they die. The tired man in Australia had been carrying the same blood flukes with him that he had picked up in Africa. These parasites were themselves at least 31 years old.