Pseudomonas aeruginosa is notorious for causing disease across vast taxonomic terrain (insects, worms, vertebrates, plants). Which viruses have a comparably wide host range?
Elio adds later:
The comments on this Talmudic Question have been both enlightening and varied. I should now pipe in with another candidate for a virus that has a particularly wide host range, the Vesicular Stomatitis Virus (VSV, Rhabdoviridae). I had long known that VSV infects mammals and insects, but then some time ago I heard Sean Whelan from Harvard mentioning that it can be induced to infect yeast as well, although this is certainly not a natural host. He supplied the following references:
The original description of the yeast experiments:
Makarow, M., L. T. Nevalainen, and L. Kaariainen (1986) Expression of the RNA genome of an animal virus in Saccharomyces cerevisiae. Proc Natl Acad Sci U S A 83: 8117-21.
The virus also replicates in C. elegans (for which there are currently no known natural viruses).
Schott, D. H., D. K. Cureton, S. P. Whelan, and C. P. Hunter (2005) An antiviral role for the RNA interference machinery in Caenorhabditis elegans. Proc Natl Acad Sci U S A 102:18420-4.
Wilkins, C., R. Dishongh, S. C. Moore, M. A. Whitt, M. Chow, and K. Machaca (2005) RNA interference is an antiviral defence mechanism in Caenorhabditis elegans. Nature 436:1044-7.