Human herpesvirus 6(HHV-6) is a human pathogen of emerging clinical significance. HHV-6 was first isolated from patients with lymphoproliferative disorders in 1986 (Salahuddin et al., 1986). HHV-6 isolates are classified into two groups as variants A(HHV-6A) and variant B(HHV-6B) (Schirmer et al., . The two variants are closely related but show consistent differences in biological, immunological, epidemiological, and molecular properties. HHV-6B is the major causative agent of exanthem subitum (ES) (Yamanishi et al., 1988), but no clear disease has yet been associated with HHV-6A.
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