Absolute requirement — What essential condition do viruses need for growth and replication?

Introduction / Context:
Viruses are obligate intracellular parasites. They do not encode complete systems for energy production or protein synthesis; therefore, they must exploit a living cell's machinery to replicate their genomes and assemble progeny virions.

Given Data / Assumptions:

• Virus replication depends on host ribosomes, nucleotides, ATP, and often host enzymes.
• Hosts can be bacteria, archaea, plants, animals, fungi, or protozoa.

Concept / Approach:
The universal requirement is not the host type but the “living cell” itself. Inert environments or cell-free media do not support complete viral replication cycles (with rare specialized exceptions requiring supplemented extracts), reinforcing the absolute need for living cellular systems.

Step-by-Step Solution:

Verification / Alternative check:
Laboratory propagation of viruses always employs living systems: cell cultures, embryonated eggs, or whole hosts, each providing active cellular processes.

Why Other Options Are Wrong:

• Bacteria/plants/animals: too narrow; many viruses infect other domains.

Common Pitfalls:
Equating “animal viruses” with all viruses; bacteriophages and plant viruses demonstrate the broader host spectrum.

Final Answer:
Living cells