Are There 10 31 Virus Particles on Earth, or More, or Fewer?

The number of virus particles on Earth is frequently reported in the scientific literature and in general-interest publications as being on the order of 10 31 , with some confusion about whether this is a high or low estimate. This number is often given without a source, although it should be attributed to a paper by Hendrix et al. published in 1999 (R. W. Hendrix, M. C. Smith, R. N. Burns, M. E. Ford, and G. F. Hatfull, Proc Natl Acad Sci U S A 96:2192–2197, 1999, https://doi.org/10.1073/pnas.96.5.2192 ).