Name the agent that causes the plague.

The agent that causes the plague is Yersinia pestis. This bacterium is a rod-shaped, gram-negative organism known for its role in significant outbreaks of plague throughout history, including the Black Death in the 14th century. Yersinia pestis is transmitted primarily through flea bites, but it can also spread through direct contact with infected animals or humans, as well as through respiratory droplets.

The pathogenicity of Yersinia pestis is largely attributed to its ability to evade the host's immune response and replicate efficiently within the host. It produces various virulence factors, including a type III secretion system that injects proteins into host cells to inhibit immune responses. This bacterium is responsible for various forms of the plague, including bubonic, septicemic, and pneumonic plague, each differing in transmission and severity.

The other options listed do not cause plague. Escherichia coli is primarily associated with gastrointestinal disorders, Staphylococcus aureus is known for skin and soft tissue infections, and Salmonella typhi causes typhoid fever. Thus, Yersinia pestis is distinctly recognized for causing the plague, making it the correct answer.