Recently the scientists Susan Scott and Christopher Duncan from Liverpool University have proposed the theory that the Black Death might have been caused by an Ebola-like virus, not a bacterium. Their rationale is that this plague spread much faster and the incubation period was much longer than the plagues caused by Yersinia pestis. (A longer period of incubation will allow carriers of the infection to travel farther and infect more people than a shorter one. When the primary vector is humans, as opposed to birds, this is of great importance.) Studies of English church-records indicate an unusually long incubation period in excess of 30 days which could account for the rapid spread, topping at 5 km/day. It also took place in completely ratless areas like Iceland. It was transferred between humans (which happens rarely with Yersinia pestis), and some genes that determine immunity to Ebola-like viruses are much more widespread in Europe than in other parts of the world.