Here's why:

* Viruses are obligate intracellular parasites. This means they cannot reproduce or carry out their life functions on their own. They rely entirely on a host cell to provide the resources they need.

* The host provides the virus with:

* Energy: Viruses use the host's cellular machinery and metabolic processes for energy production.

* Building blocks: They use the host's building blocks like amino acids and nucleotides to synthesize their own proteins and genetic material.

* Environment: The host cell's internal environment provides a suitable place for the virus to replicate and survive.

Let me know if you have any other questions!