Ragged Astronauts — Bob Shaw

"Two separate worlds are in such close proximity that they share the same atmosphere. The first, Land, is engaged in a life and death struggle with an alien species which can literally poison humans, and whose death-dealing abilities can suddenly be passed on like a plague. Overland is the second world, an apparently barren world, which may provide the only hope of refuge from the alien blight...This is Shaw's best novel yet, and that's not an insignificant accomplishment."
--Science Fiction Chronicle