iii) We should take into account the evolution of antigens (c.f. the new strains of Tuberculosis). Prevalence of human herpesvirus 6 antibodies and DNA in allogeneic stem cell transplant patients: two-year single centre experience INTRODUCTION: Human herpesvirus 6 (HHV-6) has been recognized as a potentially significant pathogen in hemopoietic stem cell transplant (HSCT) recipients. Different clinical manifestations have been described, including fever, skin rash, bone marrow suppression, and encephalitis. MATERIALS AND METHODS: A retrospective review of a group of 26 adult recipients of allogeneic HSCTs was conducted. Serum samples taken before transplant were examined for the presence of specific anti-HHV-6 IgM and IgG antibodies. After transplantation, quantitative real-time PCR was used to determine viral load in plasma samples from days 0–80 post-transplant. RESULTS: HHV-6 DNA was detected in plasma samples in 8 (30%) of the 26 recipients between days 18 and 40 after transplantation. All of them developed fever of unknown origin and over 50% had graft-versus-host disease features. Three individuals from this group died during detectable HHV-6 viremia. Another two recipients showed a single positive PCR result at a later time. Infection with HHV-6 was thus confirmed in 10 (38.5%) of the 26 graft recipients. CONCLUSIONS: There is a high frequency of detectable HHV-6 viral load in stem cell transplant recipients in Poland. Further investigation to monitor HHV-6 reactivation in graft recipients will be important to improve outcome for these patients. Human herpesvirus type 6 (HHV-6) belongs to the subfamily β-herpesvirinae and is closely related to HHV-7 and HHV-5 (formerly known as cytomegalovirus -CMV) [5] . Most people have had a primary HHV-6 infection in early childhood [3] and the virus is widespread in the human population, as shown by the presence of specific antibodies in 90% of healthy adults [17] . As a result of the primary infection, HHV-6 is assumed to establish a latent infection and viral DNA can be detected in salivary glands [7] , monocytes [10] , and early bone marrow progenitor cells [13] .