Paul Wesson. Although it is ostensibly a review of the ideas of panspermia (and a nice balanced one at that), it also gets to some rather interesting points.
The transfer of planetary - potentially life-carrying - material between stellar systems seems only likely for tiny particles. Dust grains of a few to a few tens of microns across are the ultimate solar sailors. The radiation pressure of photons from one star accelerates these minute pieces across interstellar space. Routes that lead into new solar systems result in the gentle deceleration of the dust as it plunges into the photon cloud of a star, and the possibility exists of being swept into the atmosphere of any handy planets - just as happens here on Earth. The question is what, if any, viable organisms can hold up over millions of years of interstellar transport - subject to cosmic rays and high-energy photons - to re-plant on a new world?
Wesson's discussion takes a rather interesting direction. He argues both that viral material may be prevalent, and that this type of fragmented DNA/RNA may be quite sufficient to help 'seed' life in new environments. One motivation for suggesting this is the conclusion that most transferred organisms are dead-on-arrival, there just doesn't seem to be a way that intact DNA and/or cells are going to survive their trans-galactic journeys. Another motivation is simple physics - a microscopic dust grain might barely have space for one poorly protected bacterium, but it could easily harbor a hundred much smaller viruses.
The hypothesis that there was an ancient 'viral world' here on the Earth - complete with unique viral genes that help replication - has been around for a while. While even viruses might not arrive intact after interstellar transit, this might be far less of a problem, since they play fast and loose with their molecules in the first place. It's an interesting twist to the long and checkered history of the idea of panspermia - perhaps we should be rushing cosmic dust particles from high in our atmosphere off to the labs for some quick looks for viral DNA?