The bubonic plague, which killed millions of people in medieval England, is making a return in the world, an ecology expert in Liverpool claims.
Rats carry fleas infected with the plague which can spread to humans
Research by Professor Michael Begon at the University of Liverpool revealed the deadly disease, also known as the Black Death, has re-emerged.
It has only killed 200 people since 1998, but Prof Begon said the threat is "growing" in Africa and the US.
The plague is caused by infected fleas carried by rats which spreads easily.
Prof Begon claims that while the majority of cases in the last five years have been seen in African states, there have also been up to 20 victims in the US each year.
The disease, which can kill people within a few days if not treated with antibiotics, is increasingly being passed by rodents to humans and is almost impossible to wipe out.
"Although a number of human cases of plague is relatively low, it would be a mistake to overlook its threat to humanity," Prof Begon told BBC Radio Merseyside.
"Because of the disease's inherent communicability, rapid spread, rapid clinical course, and high mortality if left untreated.
"You can't realistically get rid of all rodents in the world.
"Plague appears to be on the increase, and for the first time there have been major outbreaks in Africa."
Cases of the more dangerous pneumonic plague, which can be spread from person to person by sneezing or coughing, have also been growing since 2006.
But experts said people should be more aware of plague symptoms of black bumps, severe vomiting and high fevers.
Prof Begon has called for better research in developing countries and access to life saving drugs to combat the disease.
The first outbreak of plague swept across England in 1348-49.