It was humans, not rats, who spread the plague
According to a study, Black Death was mostly spread by humans, and not rats, in the middle of the 14th century - contrary to the popular belief that it was mainly the rodents responsible for the first wave of the deadly epidemics.
The first big outbreak killed 25 million people, more than a third of the population of Europe by that time (By https://wellcomeimages.org/indexplus/obf_images/e2/78/a40362372a6f19abb7497801c848.webpGallery: https://wellcomeimages.org/indexplus/image/L0004057.htmlWellcome Collection gallery (2018-04-05): https://wellcomecollection.org/works/awnp6vyq CC-BY-4.0, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=35866769)
Rats traditionally have a really bad reputation, partly for allegedly having been one of the main factors in the spreading of the Black Death in the Middle Ages. Though these widespread rodents - and their fleas - did contribute to the plague epidemics in 14th-19th Century Europe to some extent, the main source of new infections were actually human fleas and body lice during the first outbreak.
Plague arrived in Europe in 1347 from Asia, causing one of the most deadly epidemic in history on the continent. In the following 400 years, the disease took the lives of millions in Europe, and caused several smaller and bigger epidemics.
According to a recent study of researchers from the universities of Oslo and Ferrara, the first outbreak, which is usually referred to as the "Black Death", and lasted from 1347 to 1351 - killing 25 million people, more than a third of the population of Europe by that time - was mainly caused by human fleas and body lice. The study was published in the journal PNAS in 2018.
For the study, researchers analyzed the pattern and scale of the epidemic, using the mortality data available from outbreaks in nine European cities. The team simulated the outbreaks in the affected cities, and created three models, in which the plague was either spread by airborne transmission, rats or human fleas and body lice.
From seven of the nine cities, it was the human parasite model that seemed to match the pattern and scale of the epidemic, and it also showed the most similarity in the number of infected people. Researchers concluded that the spread of the disease would have been "slower" if it was transmitted by rats.