Certain species of rats on Christmas Island in the Pacific went extinct due to a protozoa infection brought by fleas. I would imagine that this type of thing has happened regularly in both plants and animals. You have the introduction of a pathogen that doesn't wipe out a similar species and eradicates the new species. Although this is a plant example, the American chestnut was almost completely wiped out by a fungus that was probably brought over by the introduction of Japanese chestnut trees which are more resistant.