If you give someone a bomb, or give someone a USB stick with a virus, or give someone a car with defective break, you are absolutely liable. Think about it.
If you give someone a USB stick with a virus, and you don't know about the virus, you aren't liable. Unless maybe you gave them some sort of warranty or guarantee that it was virus-free.
The lesson: don't use USB sticks people give you, unless you have your own way of verifying that they're virus-free.
Also, don't give people bombs. That's usually illegal, unlike giving someone software with unknown bugs in it.
