Tiny infectious agents that can only multiply when they are inside a host are called what?

• A. Virus
• B. Bacteria
• C. Fungus
• D. Microorganism

Answer: (A)

Viruses are tiny infectious agents that can only multiply inside a host cell. They are obligate intracellular parasites: outside a cell they’re just inert particles with genetic material wrapped in a protein coat. They lack the machinery to carry out metabolism or replicate on their own, so they hijack the host’s cellular systems to make new virus particles. This dependence on a living host to reproduce sets them apart from bacteria and fungi, which are cellular organisms capable of growth and replication independently. A broad term like microorganisms includes viruses, but it doesn’t specify this crucial trait, so it isn’t as precise. That’s why virus is the best fit for the description.