Human herpes viruses 6 (HHV-6), as its name suggests, is a member of the herpes family. There are two variants, HHV-6A and HHV-6B.As far as one can tell, HHV-6B is the more ‘common’ one of the two. It is a common cause of the viral disease roseola that is seen in over 95% of young children by the age of 2 - a self-limited fever and rash.  Much like chicken pox and shingles, the virus can be reactivated in adults that were infected as children.  The virus persists after the initial infection in a latent ('hibernating') form in the cells of the central nervous system, bone marrow and immune system and it can then reactivate and begin replicating again (Fig. 1).

HHV-6, MS and ME
The other strain, however, HHV-6A, is the one implicated in AIDS, MS and ME, amongst other diseases.  In the case of MS, a direct link has been found between it and HHV-6A [1].  In addition, Dr. Dharam Ablashi, co-discoverer of HHV-6 and Scientific Director of the HHV-6 has summarised from existing literature that when lab methods detecting the presence of active HHV-6A infection are used, an exceptionally strong, statistically significant association between HHV-6A and both multiple sclerosis and ME is consistently seen. From what I’ve found, it seems that it is not a case of HHV-6A being reactivated in an adult like I explained before but people with MS, ME etc. acquiring it as a primary infection.  So, they haven’t been infected as children etc.  Using the chicken pox ‘analogy’ again, it’s like when adults suffer from the chicken pox (not having caught it as children), and of course it can be a lot worse when this happens.  They haven’t built up immunity to it.