Squirrels can pose indirect dangers to humans and human settlements because of their ability to act as carriers for rabies and other diseases but more prominently because of their tendency to chew electrical wires. Not only can chewing these wires kill the squirrels in question, it can start dangerous electrical fires which can destroy houses, burn down surrounding trees and structures and even kill humans and other animals.
Squirrels behaving erratically may be carrying diseases which can be harmful to humans. Panicked squirrels may attack by biting and clawing and while the damage done by their claws and teeth is painful, it is not dangerous on its own. What is dangerous are the diseases which, rarely, can be spread by such contact.
Nesting squirrels will seek out sheltered places in which to build their drays. They will attempt to make these nests warm by harvesting materials from their environment, and if that environment is indoors this harvesting can disrupt electrical systems, damage insulation and cause other indoor problems.
Squirrels can also die in confined indoor spaces and cause health and hygiene problems while decomposing. It is important to take steps to keep squirrels out of the house in the first place and to help them escape if they become trapped.