Tuberculosis is caused by tubercle bacillus, which is a/an:

Tuberculosis is caused by a specific type of bacteria known as Mycobacterium tuberculosis, which is commonly referred to as the tubercle bacillus. This organism has distinct characteristics that classify it within the bacterial family. Bacteria are single-celled microorganisms that can be pathogenic, and Mycobacterium tuberculosis is an example of a rod-shaped bacterium that can lead to severe respiratory illness.

Understanding tuberculosis involves recognizing the mode of transmission and its effects on human health. The disease primarily affects the lungs, although it can spread to other parts of the body. The distinction of tuberculosis as a bacterial infection is crucial because it influences diagnosis, treatment, and public health strategies utilized to control the spread of the disease.

The other options represent different types of pathogens: viruses are acellular and require a host to replicate, protozoa are single-celled eukaryotic organisms often associated with other types of infections, and fungi are also eukaryotic but typically relate to infections such as candidiasis or athlete's foot. However, none of these options correctly describes the causative agent of tuberculosis, reinforcing that the correct identification as bacteria is essential for understanding the nature of the disease and appropriate responses to it.