What is the sickness in the last kingdom?

The sickness in The Last Kingdom is a mysterious illness that spreads throughout England, causing the deaths of many people. It is believed to be spread by rats and fleas, and its symptoms include fever, vomiting, and body aches. The illness is particularly deadly to children and the elderly, and it places a strain on the healthcare resources of the time, which were limited at best. Many people turn to traditional remedies or resort to desperate measures to try and cure themselves, but ultimately, the sickness takes a heavy toll on the population of England.