**Core Concept**
Brucellosis is a zoonotic infection caused by the gram-negative bacterium Brucella, primarily affecting animals but can be transmitted to humans. The disease is characterized by systemic symptoms such as fever, fatigue, and weight loss, often accompanied by localized symptoms depending on the site of infection.

**Why the Correct Answer is Right**
Brucella abortus is one of the most common species responsible for human brucellosis, particularly in areas where cattle are commonly infected. The bacteria primarily enter the body through contact with infected animal products, such as unpasteurized dairy or undercooked meat. The bacteria then multiply within the host's reticuloendothelial system, leading to symptoms such as fever, fatigue, and weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Brucella suis is another species of Brucella, but it is more commonly associated with swine and is less frequently implicated in human disease.
**Option B:** Brucella melitensis is a highly virulent species, but it is more commonly associated with sheep and goats rather than cattle.
**Option C:** Brucella canis is a species that primarily infects dogs, and human infections are rare.

**Clinical Pearl / High-Yield Fact**
Brucellosis can be challenging to diagnose due to its nonspecific symptoms and the need for specialized laboratory testing. A high index of suspicion is essential, particularly in individuals who have recently traveled to or worked with animals in endemic areas.

**Correct Answer: B. Brucella melitensis is a highly virulent species, but it is more commonly associated with sheep and goats rather than cattle.