Stephenson is essentially drawing on the supposition that all information, regardless of means of transmission, is the same. the metavirus snow crash is, as i understand it, a binary code translation of what was originally a biological virus, Asherah; in this particular case, the virus was a herpes simplex derivative which, as mentioned in the novel, attacks the victims nervous system. it should be noted that most viruses (asherah presumably included) can subsist without what we would think of as an actual physical "body"; once in a host, viral genetic information gets incorporated into the genome of the host and can lay dormant for years. it has been supposed that a significant percentage of the information in our genome is residual virus DNA, and so it is reasonable to assume that a virus as widespread and long-lasting as asherah exists not as an actual virus, per se, but has sustained itself ina dormant form in our genetic code.
That having been said, we translate genetic information into binary information all the time; any time a scientist looks up a gene sequence on a computer, he is essentially looking at information that has been translated from gene to binary code, and then from binary code to human written language. while just reading the genome of a virus wont effect us, stephenson posits that the natural bicameral nature of the human brain is exacerbated in hackers and programmers, who have spent years reading and manipulating binary code. this (so stephenson tells us) makes them more vulnerable not to the actual biological virus, but to the binary representation of it; because of their programming mindset, their brain has become a vector for viral information just as everyone's body is.
As to Rife picking it up off the radio, as i said before information can be transmitted in essentially any form; as long as it isn't corrupted by an incomplete translation into auditory signals, there's no reason why he wouldn't be able to reconstitute the virus's "genome" from something that was broadcast on radiowaves. granted, there has to be someone translating and broadcasting said information, but it was intimated in the novel that asherah has retained a small cult throughout the ages, and we can assume it was their doing. if you wanna know more about information transfer as independent from an actual, physical vector, i suggest looking up "memes"; some of it's been debunked at this point, but it's an interesting theory comparing the perpetuation of social or cultural information with the perpetuation and evolution of genetic traits.