The correct answer is option 1: Bacteria.

Plague, also known as the Black Death, is caused by the bacterium called Yersinia pestis. This bacterium is primarily transmitted through fleas that infest rats. When an infected flea bites a human, the bacteria can enter their bloodstream, leading to the development of the disease. Plague can manifest in different forms, including bubonic plague (characterized by swollen and painful lymph nodes), septicemic plague (infection spreads to the bloodstream), and pneumonic plague (infection affects the lungs). It is important to note that options 2 (Protozoa) and 3 (Virus) are incorrect because they do not cause plague. Protozoa are single-celled organisms, and viruses are smaller infectious agents that require a host to replicate. Therefore, the correct answer is option 1: Bacteria.