Developing vaccine for AIDS is very difficult. Give reason.

Explanation

### Step-by-Step Solution:

1. **Understanding AIDS**:
- AIDS stands for Acquired Immunodeficiency Syndrome. It is caused by the Human Immunodeficiency Virus (HIV), which attacks the immune system, specifically the white blood cells (WBCs).

2. **Impact on the Immune System**:
- The primary effect of HIV is the gradual destruction of the immune system. As the virus replicates, it kills WBCs, which are crucial for fighting off infections and diseases.

...