Bubonic plague is associated with an aversion to the urban rat, though the real public enemy is the rat’s fleas, whose bites introduce the bacterium Yersinia
pestis into the bloodstream. Deposited in lymph nodes, it causes pathognomonic “buboes” (from the Greek, meaning swelling) . The lumps expand, turn a mottled purplish-black and inflict pain bad enough, on one account, to make even the comatose writhe.
The plague is believed to be the cause of the “Black Death”, which took a scythe to Asia and Europe in the 14th century, killing more than half the population. The World Health Organisation still reports up to 2,000 annual cases of the disease, which is now, thankfully, treatable with antibiotics.