The brown rat Rattus norvegicus is a pest in Britain and in many other countries. Wikipedia calls R. norvegicus ‘the most successful mammal on the planet after humans.’ Now floods have driven it from its burrows and it’s invading people’s houses.
R. norvegicus has been selectively bred to produce pet rats and lab rats, so it’s far from being a useless animal. But in the wild it’s a pest. It eats human food and livestock feed, it damages structures by burrowing and gnawing, it carries infectious diseases including diseases which threaten livestock and zoonoses which threaten humans. It doesn’t seem to have contributed to the spread of the Black Death (bubonic plague) – that was the black ratRattus rattus – but there’s good evidence that rats on slave ships were involved in the spread of the very common zoonotic parasite Toxoplasma gondii.
The recent floods in Britain aren’t thought to have increased rat populations but they’ve made rats more visible. They need to live somewhere to survive. When flooded out, they seek new homes and those new homes can include our own homes.