Which virus is known to cause a reduction in T-cells and B-cells in the body?

The virus responsible for causing a significant reduction in T-cells and B-cells in the body is HIV (Human Immunodeficiency Virus). HIV primarily targets the immune system, specifically CD4+ T-cells, which are crucial for orchestrating the immune response. As the infection progresses, the depletion of these cells leads to a weakened immune system, making individuals more susceptible to opportunistic infections and certain cancers.

Unlike HIV, other viruses listed do not have the same direct impact on the immune system's T-cell and B-cell populations. Hepatitis B and Hepatitis C primarily affect the liver and can lead to chronic liver disease, while Hepatitis A is typically a self-limiting infection that does not have long-term effects on immune cell levels. Therefore, the distinct effect of HIV on the immune system by significantly reducing both T-cells and B-cells highlights why it is the correct answer in this context.