Left untreated, the plague can move into the lungs. This is known as pneumonic plague, which is the most serious type of the disease.
At this point, the infection can be transmitted to other humans in droplets.
There were 3,248 global cases of bubonic plague between 2010 and 2015, with most human cases occurring in Africa.
In the UK, the risk of infection remains very low, according to the latest government risk assessment.