## **Core Concept**
Haverhill fever, also known as rat-bite fever, is an infectious disease caused by specific bacteria. It is characterized by fever, headache, and pain in the hands and feet, typically following a rat bite. The condition is significant in the differential diagnosis of febrile illnesses with a history of animal exposure.

## **Why the Correct Answer is Right**
The correct answer, *Streptobacillus moniliformis*, is a Gram-negative bacterium that is one of the causative agents of rat-bite fever. This bacterium is commonly found in the mouths of rats and can be transmitted to humans through bites. The disease manifests as Haverhill fever or rat-bite fever, with symptoms including fever, chills, headache, and myalgias. *Streptobacillus moniliformis* is known for its ability to cause relapsing fevers.

## **Why Each Wrong Option is Incorrect**
* **Option A:** *Spirilla minus* is another causative agent of rat-bite fever but is more commonly associated with the classic form of the disease, which has a more acute onset and is also known as sodoku. While it causes similar symptoms, it is not the specific cause of Haverhill fever.
* **Option C:** *Salmonella Typhi* is the causative agent of typhoid fever, which presents with different symptoms and epidemiology, primarily involving contaminated food and water rather than rat bites.
* **Option D:** *Bacillus anthracis* causes anthrax, a zoonotic infection that can present in cutaneous, inhalational, or gastrointestinal forms but is not associated with rat bites or Haverhill fever.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Haverhill fever (rat-bite fever) can be caused by two main bacteria: *Streptobacillus moniliformis* and *Spirilla minus*. Clinicians should consider rat-bite fever in patients with fever and a history of rat exposure or bite, as early treatment can significantly improve outcomes.

## **Correct Answer:** B. Streptobacillus moniliformis.