Final answer:
Microorganisms, including cellular and viral, can have both harmful and beneficial effects on hosts. They are found in all three domains of life and play essential roles in various processes.
Step-by-step explanation:
Microorganisms, cellular and viral, can interact with both human and nonhuman hosts in beneficial, neutral or detrimental ways. They are found in all three domains of life (Archaea, Bacteria, and Eukarya) and are essential for life and various processes, such as biogeochemical cycles and plant/animal microbiota. While some microorganisms can cause diseases, many are beneficial and provide essential models for understanding life processes.