Actually you misunderstand me. I also think its a fine idea for them to attach themselves to a host. Though I was thinking they might attach to the thyroid system instead of the DNA itself. Maybe feeding off of hormones. Might also explain the variance in aging/regression rate amongst different races and individuals. Where by attaching to the DNA some humanoids, like Klingons and the Phange effected Vidiians, might not be suitable hosts if they break down amino acids down differently. The bugs themselves in either case could be either artifical(nano-bots) or biochemical(micro-organisms) it matters not. In one case they were programmed in a certain matter and may have evolved their programming to optimize reproduction and efficiency. In the other, They simply evolved naturally to deal with the harshness of their home planet, which may or may not be Rkar N'ra, to give them the best chance of continuing their species.