What are the three big groups of microorganisms?

The three big groups of microorganisms are bacteria, viruses, and fungi. Bacteria are single-celled organisms that can be found in various environments and can be beneficial or harmful. Viruses are much smaller and require a host cell to replicate, often causing diseases. Fungi, which include yeasts and molds, are eukaryotic organisms that play essential roles in decomposition and nutrient cycling.