Story highlights Plague is caused by infection with the bacterium Yersinia pestis

More than 1,800 people were infected with the plague last year

(CNN) One of the worst pandemics in human history, the Black Death, along with a string of plague outbreaks that occurred during the 14th to 19th centuries, was spread by human fleas and body lice, a new study suggests.

It was previously suggested that rodents, whose fleas can also transmit the bacteria behind the plague, were the main culprits behind Europe's second pandemic of the disease that saw a string of outbreaks occur in succession.

Plague is caused by infection with the bacterium Yersinia pestis and is typically spread through the bite of infected fleas, frequently carried by rats, causing bubonic plague. Symptoms include painful, swollen lymph nodes, called bubos, as well as fever, chills and coughing.

Pneumonic plague is more virulent or damaging and is an advanced form characterized by a severe lung infection. The infection can be transmitted from person to person via airborne droplets -- coughing or sneezing. Outbreaks still occur today, with an outbreak in Madagascar last year infecting more than 1,800 people.

Photos: Photos: The deer mouse is a potential source of diseases including the plague, salmonellosis and hantavirus. Hide Caption 1 of 9 Photos: Often, humans become infected after being bitten by fleas that have bitten infected animals like deer mice and voles. Hide Caption 2 of 9 Photos: Another easy way humans become infected is after their pet, mainly a house cat, hunts an infected rodent and then bites the owner or is host to infected fleas. Hide Caption 3 of 9 Photos: The first known outbreak of plague in the United States came after infected rodents from China that were on ships mingled with urban rats in port cities. Los Angeles had an outbreak of plague in 1924. Hide Caption 4 of 9 Photos: The California ground squirrel is highly susceptible to becoming infected with the plague and transmitting it widely to other squirrels. Hide Caption 5 of 9 Photos: Rock squirrels are also highly susceptible to plague bacteria and can spread it widely among its species. Hide Caption 6 of 9 Photos: Prairie dogs are some of the animals most affected by large die-offs after becoming infected with the plague. Hide Caption 7 of 9 Photos: Scientists are testing vaccines to protect the black-footed ferret , an endangered species, from getting the plague. The species has experienced devastating losses from the plague over the years. Hide Caption 8 of 9 Photos: California voles rarely come into contact with humans, since they are easily frightened, yet they can carry the plague and tularemia. Hide Caption 9 of 9

To understand the historic outbreak, scientists from the University of Oslo modeled the three transmission routes for the disease -- rats, airborne and human fleas and lice -- using mortality data for nine outbreaks that spanned the time period of the second pandemic. Their analysis found that human ectoparasites, such as fleas, reflected the death trends most accurately.

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