**Core Concept**
Toxoplasmosis is a parasitic infection caused by *Toxoplasma gondii*, a protozoan that can infect a wide range of hosts, including humans. The primary mode of transmission involves the ingestion of oocysts or tissue cysts from contaminated food or water.

**Why the Correct Answer is Right**
The correct answer is **C. Ingestion of contaminated food and water**. This route of transmission occurs when humans consume food or water contaminated with *Toxoplasma* oocysts or tissue cysts. The oocysts are shed in the feces of infected cats, which can contaminate soil, water, and other surfaces. Humans can become infected by ingesting these contaminated substances, often through undercooked meat, unwashed produce, or contaminated water. The parasite then invades the host's cells, where it can cause a range of symptoms, from mild flu-like symptoms to severe neurological problems.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while vertical transmission (from mother to fetus) is a possible route of transmission, it is not the primary mode of transmission. Vertical transmission occurs when a pregnant woman becomes infected with *Toxoplasma* and passes the parasite to her unborn child.
* **Option B:** This option is incorrect because while blood transfusions can transmit *Toxoplasma* to immunocompromised individuals, such as those with HIV/AIDS, it is not a common mode of transmission in the general population.

**Clinical Pearl / High-Yield Fact**
Toxoplasmosis is more likely to cause severe symptoms in immunocompromised individuals, such as those with HIV/AIDS or undergoing chemotherapy. Pregnant women should also take precautions to avoid infection, as vertical transmission can lead to serious complications in the fetus.

**Correct Answer: C. Ingestion of contaminated food and water**