**Core Concept**
Brucellosis is a zoonotic infection caused by the genus *Brucella*, with *Brucella abortus* being one of the most common species. It is primarily an occupational disease affecting individuals who work with infected animals. The bacterium *Brucella abortus* is typically transmitted through contact with infected livestock.

**Why the Correct Answer is Right**
*Brucella abortus* primarily affects cattle, leading to abortion in pregnant females. The bacteria can be transmitted to humans through direct contact with infected animals or their products, such as unpasteurized milk. In humans, *Brucella abortus* infection can cause undulant fever, fatigue, and swelling of the lymph nodes.

**Why Each Wrong Option is Incorrect**
**Option A:** Not the primary host, although other animals can be infected.
**Option B:** Not the primary host, *Brucella melitensis* affects goats and sheep.
**Option D:** Not the primary host, *Brucella suis* affects pigs.

**Clinical Pearl / High-Yield Fact**
*Brucella abortus* infection is a classic example of a zoonotic disease, emphasizing the importance of occupational hygiene and vaccination of livestock to prevent human infection.

**Correct Answer:** D. Cattle.