Which of the following diseases is caused by a virus?
A. Tuberculosis
B. Typhoid
C. Polio
D. Diphtheria

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Hint: Diseases are caused by bacteria, viruses, protozoans, fungi, and helminths. Antibiotics used to treat bacterial infections are not effective against viral infections. This is because viruses do not have their metabolic machinery to serve as a target for the antibiotics.

 Complete answer:
To answer this question, we should know the causative agents of the given diseases.
Let’s first discuss some of the possible categories of the pathogens. The causative agent of a disease is called a pathogen.
For example, bacteria, the prokaryotic microbes with no well-defined nucleus cause several diseases in humans and animals. Mycobacterium tuberculosis is a bacterium that causes tuberculosis, a disease transmitted by air. Similarly, typhoid is a waterborne disease caused by a bacterium called Salmonella typhi. Diphtheria is also caused by bacterial species called Corynebacterium diphtheriae.
Besides bacteria, viruses are also called several diseases in humans. They are distinct from bacteria and other pathogens as they lack their metabolic machinery and are acellular. A typical structure of a virus includes a protein coat called a capsid and the genome present inside it. As their genome, viruses can have single-stranded or double-stranded RNA or DNA.
Polio is a disease caused by the poliovirus. Poliovirus has a single-stranded RNA as its genome and mainly targets the cells of gut and neurons of the spinal cord and therefore, cause gastrointestinal problems or paralytic attack.

So, the correct answer is C. Polio.

Note: We should know that although viruses cause diseases, they are obligate parasites and require a host cell to exhibit the life processes. Therefore, viruses are not categorized as prokaryotes or eukaryotes.