It was therefore a pneumonic plague rather than a bubonic plague. The infection was transmitted from human to human, rather than through rat fleas that bit a sick person and then bit another victim. "As an [rat fleas] explanation for the Black Death by itself, it's simply not good enough. It can't spread fast enough from one household to another to cause the huge number of cases we saw during the Black Death." epidemics,” said Dr Tim Brooks, a Porton Down scientist who was not part of the Crossrail team, who will present his theory in a Channel 4 documentary, Secret History: The Return of the Black