The simplest type of microorganisms that can only replicate themselves in the living cells of their hosts. They do not respond to antibiotics.

• A. Bacteria
• B. Fungi
• C. Viruses
• D. Protozoa

Answer: (C)

Viruses are the simplest type of microorganism that can only replicate inside living cells of a host; they lack independent metabolic machinery, so they cannot grow or reproduce on their own. Because antibiotics target bacterial processes—such as cell wall synthesis or bacterial ribosomes—they don’t affect viruses, which don’t have those targets. Bacteria can multiply outside cells and are commonly inhibited or killed by antibiotics, while fungi and protozoa are different types of organisms that require other kinds of drugs. A practical example is a common cold virus—antibiotics won’t cure it because the virus replicates inside host cells.