**Core Concept**
The transmission of **Human Immunodeficiency Virus (HIV)** can occur through various routes, including sexual contact, blood transfusion, and mother-to-child transmission. Understanding the different modes of HIV transmission is crucial for developing effective prevention strategies. The virus attacks the body's immune system, specifically the **CD4 cells**.

**Why the Correct Answer is Right**
The correct answer is related to the least common mode of transmission, which involves casual contact. This includes sharing food, utensils, or clothing, and even biting or spitting. However, HIV is not transmitted through **saliva**, **sweat**, or **tears** unless they are mixed with the blood of an infected person.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because mother-to-child transmission is a recognized mode.
**Option B:** Incorrect as blood transfusion is a known route.
**Option C:** Incorrect because sexual contact is a common mode.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that HIV is not transmitted through casual contact, which is often a concern for those unaware of the virus's transmission modes.

**Correct Answer:** D. Casual contact.