1). Poor hygiene
2). Unsanitary drinking water
3).Sleeping on beds made of straw that wouldn’t be changed for months at a time and would have rat urine and rats living in allowing for the spread of the plague
4). Infestation of infected rats through ports. (Italy is one example that became infected, through rats coming off of ships into their city with the plague)
5). No proper medical knowledge: they were performing bloodletting + also not wearing any face coverings
6). The plague doctors, the ones that didn’t show any symptoms usually had some form of AIDS, and therefore were asymptomatic and actually transferring it to more people that they were helping