It describes a British man of about 30 who, as a toddler, was given the full course of polio vaccines in use at the time—and who, for 28 years, has been harboring mutated, virulent poliovirus in his guts and shedding them in his feces. The researchers who wrote about him call him, and other patients like him, “an obvious risk to the eradication program.”

Long-term shedders of poliovirus have been identified before. It’s a rare phenomenon, and for it to occur, several things have to happen: Someone has to be vaccinated as an infant with the oral polio vaccine, which contains a weakened live virus; has to have an immune-system deficiency that keeps the body from eliminating the vaccine virus after immunity develops; and, usually, has to live in an industrialized country where medical care is good enough to keep children with immune-system deficiencies alive.

That has happened a number of times—more than 70—since polio vaccination began in the 1950s, but only a few people shed virus long enough to make public health planners concerned. This man is the longest-duration shedder known, but he may not be the only one. Sampling programs that look for live poliovirus in sewage have identified mutated vaccine viruses in Finland, Israel and eastern Europe, indicating other, unidentified shedders may be out there too.