Squirrels are more afraid of humans than humans are of them. However, their fear can cause them to react violently. While it might be ok to get within 10 feet of a squirrel in an outdoor setting, you don't want to corner one in your house. It's not safe to pick them up, pet them, feed them people food, or treat them like domesticated animals. Squirrels will attack dogs if they get too close or make them feel fearful for their safety.
They do carry diseases like rabies, bubonic plague, intestinal worms, parasites, fleas, lice, and other undesirable contagions. Squirrels' teeth never stop growing. They're sharp, long, and strong enough to inflict quite a bit of damage. Many humans have been bitten by squirrels and contracted bacterial infections in their wounds. It's best to stay out of their safe zone whenever possible.