Here's why:

* Viruses are obligate intracellular parasites: This means they can only replicate inside a living cell.

* Viral Replication Cycle: When a virus infects a cell, it inserts its genetic material (DNA or RNA) into the host cell. This genetic material then hijacks the cell's machinery:

* Protein Synthesis: The cell's ribosomes are forced to produce viral proteins, which are essential for building new viruses.

* Genetic Material Replication: The cell's DNA polymerase or RNA polymerase is used to copy the viral genetic material.

* Assembly: The newly made viral proteins and genetic material self-assemble into new virus particles.

* Release: The new viruses then burst out of the cell (lytic cycle) or bud off (budding), spreading the infection.

So, when a virus is active, the cell is essentially forced to work against its own best interests, producing the components needed for new viruses. This is why viral infections can cause so much damage.