Peter Parker was bit by a radioactive spider which was supposed to have mutated his DNA(as we know radiation and other factors can induce mutation in living things).
I think that story may have been retconned, tho. Not entirely certain.
I think that story may have been retconned, tho. Not entirely certain.

After a day or two without food and water, they'd be incapable of killing someone, and even in numbers, they'd be very easy to take down.