In which of the following disease, the number of `T_H` cells in body considerably decreases over the period of time ?

Options

A. AIDS
B. Cancer
C. Malaria
D. Typhoid

Answer

To solve the question, "In which of the following diseases does the number of T_H cells in the body considerably decrease over time?" we need to analyze each option provided.

### Step-by-Step Solution:

1. **Understanding T_H Cells**: T_H cells, or T helper cells, are a type of white blood cell that play a crucial role in the immune system by helping to activate other immune cells. Their count is an important indicator of immune health.

2. **Analyzing the Options**:
- **AIDS**: Acquired Immunodeficiency Syndrome (AIDS) is caused by the Human Immunodeficiency Virus (HIV), which specifically targets and destroys T helper cells (CD4 cells). Over time, the number of these cells decreases significantly, leading to a weakened immune system.
- **Cancer**: While cancer can affect the immune system, it does not specifically target T helper cells. The immune response may be altered due to the presence of tumors, but T helper cell counts do not necessarily decrease in a consistent manner.
- **Malaria**: This is an infectious disease caused by the Plasmodium parasite. While malaria can impact the immune system, it does not lead to a significant decrease in T helper cells over time.
- **Typhoid**: Caused by the bacterium Salmonella typhi, typhoid fever can affect the immune response but does not lead to a significant decrease in T helper cells.

3. **Conclusion**: Based on the analysis, the disease in which the number of T_H cells considerably decreases over time is **AIDS**.

### Final Answer:
**AIDS**