**Core Concept**
Toxoplasma gondii is an obligate intracellular parasite that primarily infects domestic and wild cats, serving as the definitive host. This unique relationship between the parasite and its host is essential for the parasite's life cycle.

**Why the Correct Answer is Right**
Toxoplasma gondii completes its life cycle in felines, where it undergoes sexual reproduction and forms oocysts. These oocysts are shed in the cat's feces and can contaminate the environment, posing a risk to other animals and humans. This cycle is crucial for the parasite's survival and transmission. The parasite can also infect other hosts, including humans, but the cat serves as the primary reservoir.

**Why Each Wrong Option is Incorrect**
**Option A:** Rabies is transmitted through the saliva of infected animals, typically through bites. Dogs, bats, and other mammals can act as reservoirs for rabies, but cats are not the primary reservoir.
**Option C:** Streptocerca infection, caused by the parasitic worm Streptocerca, is transmitted through the bites of infected flies. This parasite does not have a specific reservoir related to cats.
**Option D:** Plague, caused by Yersinia pestis, is transmitted through the bites of infected fleas or direct contact with infected animals. While cats can be infected with plague, they are not the primary reservoir.

**Clinical Pearl / High-Yield Fact**
Toxoplasma gondii infection during pregnancy can lead to congenital toxoplasmosis, a condition that can cause serious health issues in the developing fetus.

✓ Correct Answer: B. Toxoplasma gondii