I think a VCO will cover a large band of frequencies while still having the square wave output. I will have to think on how long to give the sweep through the frequencies. Like 50KHz per square wave, which depending on the upper end of frequencies used may only take 10 to 15 seconds to sweep through them. You still need enough voltage to get through the skin to get to the bugs/virus/bacteria. Whats the highest frequency needed? The 555 is stable at 500KHz but some can go to 1MHz.