What is the term for an organism that takes over an entire cell?

The term for an organism that takes over an entire cell is a virus. Viruses are unique infectious agents that cannot reproduce independently; instead, they require a host cell to replicate. When a virus infects a cell, it enters the host's cellular machinery and co-opts it to produce more virus particles, often leading to cell damage or death in the process. This characteristic of needing to hijack a host cell's functions to propagate is a defining feature of viruses, distinguishing them from other organisms such as bacteria, fungi, and protozoa, which have different modes of existence and reproduction.