What virus interferes with the body's ability to fight infections?

HIV, or Human Immunodeficiency Virus, is the correct answer because it specifically targets the immune system, particularly the CD4 cells (or T-helper cells) that are crucial for mounting an immune response against infections. When HIV infects these cells, it impairs the body’s ability to fight off various infections and diseases, which can lead to acquired immunodeficiency syndrome (AIDS) if not managed appropriately.

In contrast, while the flu virus, herpes virus, and hepatitis C can have significant health impacts, they do not fundamentally weaken the immune system in the same way. The flu virus can cause illness and may lead to secondary infections, but it is the immune response to the flu that ultimately helps to clear the virus. The herpes virus can establish latent infections and cause outbreaks, but it does not specifically target and destroy immune cells. Hepatitis C primarily affects the liver and can lead to severe liver disease, not a direct compromise to overall immune function. Thus, HIV is unique in its direct attack on the immune system, making the body's ability to fight infections severely compromised.