I am not sure if the 30,000 has anything to do with it, but I do lean towards the 2500 Hz range for colds and flu. I do suspect that there may be times that the body can be so overwhelmed with a viral infection that there is just not much that can be done. But on the other hand, it is hard to say that it would not have been much more serious if the zapper had not been used.
Also, I will say that the most important thing to do to keep colds and flu down is to keep vitamin D up.