Background. Recently, it has been suggested that human herpes virus 6 (HHV6) may play a role in the pathogenesis of multiple sclerosis (MS). Our purpose is to determine the incidence of reactivated HHV6 in MS patients. Methods. Viral sequence analyzed by qPCR in the peripheral blood mononuclear cells (PBMCs), serum, and saliva samples of different subtypes of MS patients ( ) and healthy controls ( ). HHV6 IgG and IgM antibody levels measured by ELISA technique in the plasma samples of both groups. Likewise, cerebrospinal fluid (CSF) samples of some MS patients ( ) were analyzed for viral sequence. Results. Results demonstrate increased levels of anti-HHV6-IgG (78.2% versus 76.4% in controls; ), and IgM (34.6% versus 6.5% in controls; ) in MS patients. Furthermore, RRMS and SPMS patients showed relatively higher anti-HHV6 IgG and IgM compared to PPMS ( ). Moreover, load of cell-free viral DNA was higher in RRMS and SPMS patients and detected in 60.2% (47/78) of MS patients, compared with 14.6% (18/123) of healthy controls ( ). Moreover, load of cell-free viral DNA was higher in RRMS and SPMS patients and detected in 60.2% (47/78) of MS patients, compared with 14.6% (18/123) of healthy controls ( ). Conclusions. The results extend the observation of an increased frequency of systemic reactivated HHV6 infection in MS patients with developed stages of disease. 1. Introduction Human herpes virus 6 (HHV-6) belongs to the beta-herpes virus subfamily of the Herpesviridae family, with a linear double-stranded DNA genome of 160？kb [1]. After primary infection, HHV-6 remains latent in lymphocytes and monocytes unless the immune system is compromised, whenever the virus reactivates [2–4]. Multiple sclerosis (MS) is the most prevalent demyelinating disease among young adults, affecting many people in developing countries [5]. The prevalence of MS, according to World Health Organization reports (2006), should be about 18 to 175 in 100,000, according to geographic distribution of the disease. However, a recent study has shown that Iran could be considered as an area with a medium to high risk of MS and in southeastern Iran the incidence rate is showing a faster growth rate, compared to previous years [6–8]. Relapsing Remitting MS (RRMS) is the most frequent (85%–90%) forms of MS and affects women about twice as often as men. Most RRMS patients develop Secondary Progressive MS (SPMS) later. About 10%–15% of patients present with insidious disease onset and steady progression, termed Primary Progressive MS (PPMS). It is not clear which factors are responsible for the
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