Rats are listed as a public health pest for good reason
If they are emerging into your living areas (i.e. you are finding droppings, seeing rats) then they will be spreading bacteria via their bodies, urine and droppings onto your floors, kitchen cupboards and work surfaces – Google away to frighten yourself on the full range of potential diseases!
The most significant risk is Weils disease, which although is only carried by a small percentage of rats, causes flu-like symptoms in humans that can result in death if left untreated
Under 5’s and over 60’s are generally most at risk from rat bourne diseases
If they are only within the building fabric – i.e. behind walls, under floors, in roof voids – then they will very actively gnaw electrical wiring, plastic pipes and structural timbers
Modern consumer units are all RCD protected these days so gnawed wires are much less likely to cause fires but you will still get lights, sockets and other such aspects of your fixed electrical circuit suddenly not working and requiring costly repairs
The real risk is plastic waste pipes and modern ‘Speedfit’ piping – damage to these can cause serious floods and water damage
Also if they are in the building fabric, then it is only a matter of time before they begin to emerge into your living areas…