Q: Given a scientific passage and an answer, generate a question for the given answer.
Passage: Viruses were first described by Dmitri Ivanovsky in 1892. He described a "non-bacterial pathogen" infecting tobacco plants. This was soon followed by the identification of the tobacco mosaic virus by Martinus Beijerinck in 1898. Since then, about 5,000 viruses have been described in detail, although it is believed that there are millions of different types. Viruses are found in almost every ecosystem on Earth, and are the most abundant type of biological entity. Viruses can be classified with a taxonomic structure from order to species. No kingdom classification exists. Viruses, not being made of cells, do not fall into any of the six prokaryotic or eukaryotic kingdoms. Answer: viruses
A:
What is the most abundant type of biological entity on the earth?