Rats acted as vectors as they carried the infected fleas into the cities. One factor that influenced the spread was that in 1350 hygiene was inadequate and often food and faeces were left in the streets this meant that the cities ect was teeming with rats as they had an appropriate food source and habitat. Humans lived alone side rats but since the rats carried the fleas the fleas would bite the people who lived there thus infecting them. The next factor is that the rats began to die of the plague so household pets or other animals started feeding on them thus becoming infected. Since pets were becoming more common they had access to the household. These pets would then bite or scratch or perhaps carry the infected fleas and pass the infection