These viruses have always been baffling, even to virologists who knew about them. Everyone assumed that they could only reproduce if all the segments infected the same host cell. But the risk of losing a piece, and so dooming the others, skyrockets as the number of pieces goes up. In 2012, two researchers calculated that the odds of successfully getting every segment in the same cell become too low with anything more than three or four segments. FBNSV, with its eight segments, “should never have evolved,” Blanc says. Its mere existence suggests “that something must be wrong in the conceptual framework of virology.”

Perhaps, he realized, these viruses don’t actually need to unite their segments in the same host cell. “If theory was saying that this is impossible, maybe the viruses just don’t do it,” he says. “And once we had this stupid idea, testing it was very easy.”