Another question that sparked attention in the New York case was the unusual dual-tropic nature of the patient's virus. In almost all cases of recent HIV infection, the acquired virus uses a protein on the surface of cells, the CCR5 chemokine receptor, to gain entry into these cells (5). This variant is therefore common in earlier-stage HIV disease. It is transmitted even if the source is infected with a mixture of HIV variants, some that use the CCR5 receptor and others that use another protein, the CXCR4 receptor. Most people infected with HIV that uses the CXCR4 receptor (as in the New York case) have been infected for a considerable period of time. Instances of initial infection with the CXCR4 variant are rare and are seen only in persons who lack the gene for the CCR5 receptor (6), which was apparently not the situation in the New York case. The case was, in fact, even more unusual because the patient had a virus that could use either type of receptor to infect cells. These “dual-tropic” strains seem to prefer to use the CXCR4 receptor in vivo (7–8) and are therefore associated with either a more advanced disease stage or a higher rate of clinical progression (9). Although seen in more advanced stages of HIV disease, however, viruses that use CXCR4 seem less transmissible than the common CCR5 form. Therefore, the type of HIV in the New York case is more consistent with a long-standing infection, not a recent one, or is the result of genetic susceptibility to infection by this unusual virus. Either explanation, if true, would decrease the public health significance of the reported case. If not a recent infection, the virus would resemble many others that have developed drug resistance in patients with advanced-stage disease. If the rapid appearance of the dual-tropic virus reflects a unique host susceptibility, others might not be readily infected after exposure.