Viruses, in general, receive mostly unfavorable publicity. However, only a small percentage of human or plant viruses actually cause disease. Many other viruses, mostly unnamed, persist in the genomes of all living beings without causing harm. Experiments show that many viral infections in plants are doing an incredible amount of good. In all of the cases examined, plant virus infections conferred drought tolerance to the host.
A more complicated microbe association involves grass plants growing in the thermal soil at Yellowstone National Park. A mutualistic fungus was found to be absolutely necessary for the plant’s heat tolerance. But the fungus itself is infected with a virus. If the virus is removed, neither the fungus nor the plant can survive the heat.
Proceedings of the AAAS annual meeting, February 2013, Boston MA