The present invention provides a novel drug useful for fundamental treatment and prophylaxis of Alzheimer's disease (AD).As a pharmaceutical composition for treating or preventing AD, an agent comprises a compound having antiviral activity against HHV-6A, HHV-6B and / or HHV-7, particularly foscarnet, as an active ingredient is used. The agent not only stopsthe progression of AD but may also be used for fundamental treatment.In addition, by using the DNA or protein originated from HHV-6A, HHV-6B and/or HHV-7 existing in the cerebrospinal fluid (CSF), plasma, blood, or in the saliva of a subject, or using an antibody or a fragment thereof specific for HHV-6A, HHV-6B and/or HHV-7 as an AD biomarker, it is possible to diagnose the risk of developing AD or the progression of AD, or to confirm the therapeutic effect of an antiviral agent used in treating AD.本發明係提供有用於阿茲海默症(AD)的根本性治療及預防的新藥。作為治療或預防AD用的醫藥組成物者，係使用以對於HHV-6A、HHV-6B及/或HHV-7具有抗病毒活性的化合物(特別是膦甲酸)作為有效成分的藥劑。該藥劑不只中止AD的進展，更有成為根本性治療手段的可能性。再者，將源自被驗者的腦脊髓液(CSF)、血漿、血液或唾液中的HHV-6A、HHV-6B及/或HHV-7的DNA或蛋白質，或HHV-6A、HHV-6B及/或HHV-7特異性的抗體或其片段作為AD生物標記，藉此可診斷罹患AD的風險或AD的進展狀況，或可確認使用於AD的治療或預防的抗病毒劑的治療效果。
