Viruses lack the machinery to reproduce on their own.

Here's why this is so important:

* Cells are self-sufficient. They have their own DNA or RNA, ribosomes to build proteins, and the ability to generate energy. They can independently grow, reproduce, and carry out all the functions of life.

* Viruses are essentially "packages" of genetic material (DNA or RNA) wrapped in a protein coat. They lack the essential components to replicate on their own. Instead, they hijack the machinery of a host cell to make more viruses. This process is called viral replication.

In other words, viruses are essentially parasites that rely on living cells to survive and reproduce.