What disease is characterized by the weakening of the immune system due to a viral infection?

The disease characterized by the weakening of the immune system due to a viral infection is HIV/AIDS. This condition is specifically caused by the Human Immunodeficiency Virus (HIV), which attacks and destroys the body's immune cells, particularly CD4 T lymphocytes. Over time, without appropriate treatment, HIV can progress to Acquired Immunodeficiency Syndrome (AIDS), a stage where the immune system is severely compromised, making the individual more susceptible to opportunistic infections and certain cancers.

HIV specifically targets the immune system’s functionality, leading to a gradual decline in the body's ability to fend off infections. This distinguishes it from the other conditions listed. For instance, hepatitis primarily affects the liver and is caused by viral infections such as hepatitis B or C but does not directly weaken the immune system in the way HIV does. Diabetes is a metabolic disorder involving insulin regulation and does not impair the immune system directly like HIV. Cancer can affect the immune system depending on the type and treatment but is not primarily a viral infection. Thus, the unequivocal association of HIV/AIDS with a viral infection that compromises immunity makes it the correct answer.