Tailed phages are a kind of bacteriophage— viruses that infect bacteria. They are the most abundant organism on Earth, with an estimated 10^31 particles in our biosphere.
If they were laid end to end, they would extend for 200 million light years! They would also outweigh the world’s population of elephants by 1000-fold. (From Virus Taxonomy, by Fauquet, Mayo et al., Elsevier, 2005)