Why are rats harmful?

They carry disease, and if you have them at home (non pet rats) then they basically pee and poo everywhere they run. They also chew on electric wires.

First of all they contaminate the environment around you, eat up ur shoes, food, clothes etc and leave their feces and pathogenic bacteria behind...

They can cause diseases like rat bite fever

HPS

leptospirosis etc......

Rats are dangerous because they carry a flea within themselves known as the rat flea.

When this flea bites a person, a deathly disease known as plague is transmitted. This disease was in rampage in the olden times and had eradicated most of the world population. It is believed that more people had died due to plague than in the World Wars.

Also, rats are pests and destroy all food items and furniture.