Host dependence: Which microbes are most strictly dependent on a living plant or animal host for replication and survival?

Introduction / Context:
Obligate intracellular dependence separates viruses from cellular life. This concept underlies antiviral strategies that target steps unique to virus-host interactions.

Given Data / Assumptions:

• Viruses lack ribosomes and metabolic machinery for independent replication.
• Many bacteria, fungi, and algae can live and reproduce in the environment.
• Some parasites are obligate intracellular, but viruses are universally dependent.

Concept / Approach:
Identify the class of microbes that cannot replicate without commandeering host cell processes. Only viruses fit this requirement universally.

Step-by-Step Solution:
Assess each group’s autonomy. Confirm that viruses lack translation and energy generation systems. Select “viruses”.

Verification / Alternative check:
Culture techniques: viruses require living cells; bacteria and fungi can be grown on artificial media.

Why Other Options Are Wrong:
Bacteria, fungi, algae, and many protozoa can grow independently under suitable conditions.

Common Pitfalls:
Confusing obligate intracellular bacteria (e.g., Chlamydia) with the universal dependence characteristic of all viruses.

Final Answer:
viruses.