A virus is defined as a sub-microscopic infectious agent that replicates inside living host cells.

A virus is defined as a sub-microscopic infectious agent that replicates inside living host cells.

• A. Living host cells
• B. In water
• C. In air
• D. On non-living surfaces

Answer: (A)

Viruses require living host cells to replicate. They are sub-microscopic infectious agents that hijack a cell’s machinery to make new viral copies. Because of this dependency, replication can't occur in water, air, or on non-living surfaces—those environments may carry viruses briefly, but they don’t support viral multiplication. This is why the defining concept is that viruses replicate inside living host cells.

Viruses require living host cells to replicate. They are sub-microscopic infectious agents that hijack a cell’s machinery to make new viral copies. Because of this dependency, replication can't occur in water, air, or on non-living surfaces—those environments may carry viruses briefly, but they don’t support viral multiplication. This is why the defining concept is that viruses replicate inside living host cells.