没必要太担心小松鼠...

Squirrels are typically not important vectors of disease - I don't think there's any documented cases of rabies transmission from squirrels, for example. Like all animals, squirrels can carry parasites, they leave behind excrement, and they can leave both in your attic. Squirrel droppings, like pretty much any wildlife dropping, are associated with Leptospirosis and Salmonella.