I've known about Rife machines for about 20 years, but I still don't know enough about them to render an opinion. The theory is that if you bombard pathogens or cancer cells with sound of a certain frequency, they'll vibrate at the resonant frequency and shatter like glassware that's subjected to high pitched sound. This is one of those theories that should be very easy to test in the laboratory; I'm not sure that anyone's actually performed lab experiments to test these machines. The American Cancer Society says they don't work but the AMA didn't back up their position with any studies in their 2008 position paper. I do know that ultrasonic devices are used to control algal growth in ponds, so the theory is plausible but absent any clinical studies, I don't think anyone can know for sure if they work.