This virology [blog] discusses estimates of the number of mammalian viruses and the costs of 'discovering' 85% of them.
My question is whether this is not a forlorn hope. The ".632 rule" in statistics says roughly that as we approach n random samples from a large population of size n we will see only about 62.3 per cent of the population. After a point we would begin to see the same viruses again and again. This argument is probably stronger with respect to marine viruses, as they may be even more numerous and sampling at depths could be difficult.
I wonder if someone with experience sampling small organisms from large populations or familiarity with the literature has an idea about the plausibility of seeing 85% of a large population, such as mammalian viruses, by sampling?
If the samples are really random (in some sense) and the population is large, a few computer simulations reveal the power of this rule of thumb.
Edit in response to comment/question:
Like any such estimate, the blog's estimate of 3.6 million is a guess. I am not reproducing the calculation here because it's just speculation.
Having said that, if 3.6 million were correct, we would have to draw about 6.8 million samples to find 85% of the existing viruses, 8.2 million to find 90%, and so on. As the blog notes, the PCR approach detects viruses similar to those we know, so the sampling is worse than random. If we look at the difficulty of finding/using a method that has a reasonable chance of capturing any of the extant viruses, the number 3.6 million looks very big.