- HHV6- HHV6 reactivation will be defined as detection of elevated HHV6 levels by PCR in blood while disease will be defined as detection in multiple sites or in one site with symptoms. Ganciclovir, Cidofovir, and foscarnet have variable in vitro activity against HHV-6, and may have a role in treating HHV-6-associated disease - hence one or more of these agents will be added in patients with disease.