What are single-celled organisms that can live independently or as parasites known as?

Single-celled organisms that can live independently or as parasites are classified as bacteria. Bacteria are one of the most fundamental forms of life and are characterized by their cellular structure. Unlike viruses, which require a host to reproduce and cannot live independently, bacteria have the ability to thrive on their own in various environments, including nutrient-rich settings.

Many bacteria play crucial roles in ecosystems and human health, such as in digestion or nutrient cycling, while some can act as parasites, causing diseases in hosts. The ability of bacteria to exist freely or within hosts underlines their diverse nature and adaptability.

Other options like viruses are not classified as living organisms since they cannot survive without a host. Parasites generally refer to organisms that live on or in a host, often harming it, but this term more broadly includes various forms of life and is not exclusive to single-celled organisms. Fungi, on the other hand, are primarily multicellular (with some exceptions) and have a different structure and life cycle than bacteria. Hence, bacteria are the most accurate answer for this question based on the characteristics described.