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How can one harmless retrovirus cause over forty different diseases?

You sound like someone who has studied bio/biochem so bear with me because I've not. Your statement leaves out the impact of the retrovirus on T4 cells. In fact I've never read that the retrovirus was responsible for AIDS per se, but that it did in some manner cause the destruction T4 cells, and hence the immune system, that resulted in the mutiplicity of diseases one sees in someone with AIDS. T4 cell < 500 --> inefficient immune system --> multiple disease state.

Are you saying this is not correct, and if so, would you explain.

HIV disease is characterized by a gradual deterioration of immune function. Most notably, crucial immune cells called CD4+ T cells are disabled and killed during the typical course of infection. These cells, sometimes called "T-helper cells," play a central role in the immune response, signalling other cells in the immune system to perform their special functions.

A healthy, uninfected person usually has 800 to 1,200 CD4+ T cells per cubic millimeter (mm3) of blood. During HIV infection, the number of these cells in a person's blood progressively declines. When a person's CD4+ T cell count falls below 200/mm3, he or she becomes particularly vulnerable to the opportunistic infections and cancers that typify AIDS, the end stage of HIV disease. People with AIDS often suffer infections of the intestinal tract, lungs, brain, eyes and other organs, as well as debilitating weight loss, diarrhea, neurologic conditions and cancers such as Kaposi's sarcoma and lymphomas.

Most scientists think that HIV causes AIDS by directly killing CD4+ T cells or interfering with their normal function, and by triggering other events that weaken a person's immune function. For example, the network of signalling molecules that normally regulates a person's immune response is disrupted during HIV disease, impairing a person's ability to fight other infections. The HIV-mediated destruction of the lymph nodes and related immunologic organs also plays a major role in causing the immunosuppression seen in people with AIDS.