The frequencies shift up and down the spectrum as the organisim adjusts to changes in it's environment. The frequencies answer to metabolic activities inside it's cells- the replication of DNA, the changes of the mitochondria as it makes RNA and metabolizes nutrients, and the movement of protiens up and down the cytozome as transcription molecules are carried to the different parts of the cell.
It's not frequencies, but quantum activities in a four-dimensional plane (and the planes higher than those, as the cells react to influences on the spiritual side of things).
This is why EMF from jarring electronics can make us sick. Extremophiles do better in high-energy environments, and sometimes EMF can excite their metabolisim. (Think of the moulds that like to feed off and aggregate minerals in the soil, like the ones that create veins of gold and silver. Their activities are safe in soil, but toxic in a body).
Yes, the theory of frequencies killing an infection via resonance is still valid, but you need to cover a spectrum of frequencies to kill efficiently over a longer period, because of the rule of genetic adaptation. Bacterium and the like have a span from generation to generation of between 2-4 days, and larger parasitic organisims of 14-28 days.
The zapper vs rife debate can allways rage plenty, but it really helps to read. :)