The uncertainty is high: The definition of a microbial species, and the difficulty of culturing them contributes to making the estimate on the total number of microbial species difficult. References: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3160642/ https://www.nature.com/articles/nrmicro2644
If you do not know which parasite to look for, then it can be difficult to identify them. Some parasites can be diagnosed entirely by looking at the symptoms, and a microscopic analysis is not necessary.
In this episode, I will explain the naming of organisms. A basic understanding of this can prove to be quite useful also for the identification of microorganisms under the microscope.