Since its discovery in 1986, HHV-6 has been associated with a wide array of clinical conditions—many of which are listed in the menu at right. While the relationship between HHV-6 infection and some of these diseases are well established, the role of HHV-6 in many other conditions remains unclear. In addition to causing “acute” disease such as encephalitis, HHV-6 can also persist as a chronic infection, nearly undetectable by most current diagnostic tests. This subacute form of HHV-6 is likely to contribute to the pathology of many diseases associated with HHV-6. There is an urgent need for more sensitive diagnostic assays and studies that can prove or disprove the important disease associations that have been suggested.