Under the IHR (1969), only cholera, plague and yellow fever remain notifiable, meaning that States are required to notify WHO if and when these diseases occur on their territory.

The original International Health Regulations agreed in 1969 were designed to help monitor and control four serious infectious diseases - cholera, plague, yellow fever and smallpox.

"Without a doubt, the plague pathogen known today as (yersinia) pestis was also the cause of the plague in the Middle Ages," he added.

The authors point out that the plague is extraordinarily adaptable.

In 2004, a Chinese survivor described to the Guardian how his home on Zhejiang province, south-east China, had been attacked by plague-inflected fleas dropped by Japanese occupation forces.