What would it take to describe the global diversity of parasites?

How many parasites are there on Earth? Here, we use helminth parasites to high-light how little is known about parasite diversity, and how insufficient our current approach will be to describe the full scope of life on Earth. Using the largest database of host-parasite associations and one of the worlds largest parasite collections, we estimate a global total of roughly 100,000 to 350,000 species of helminth endoparasites of vertebrates, of which 85% to 95% are unknown to science. The parasites of amphibians and reptiles remain the most poorly described, but the majority of undescribed species are likely parasites of birds and bony fish. Missing species are disproportionately likely to be smaller parasites of smaller hosts in undersampled countries. At current rates, it would take centuries to comprehensively sample, collect, and name vertebrate helminths. While some have suggested that macroecology can work around existing data limitations, we argue that patterns described from a small, biased sample of diversity arent necessarily reliable, especially as host-parasite networks are increasingly altered by global change. In the spirit of moon-shots like the Human Genome Project and the Global Virome Project, we consider the idea of a Global Parasite Project: a global effort to transform parasitology and inventory parasite diversity at an unprecedented pace.