A member of the herpesvirus family; best known for causing roseola in babies. More than 95% of the population has what's called the "B variant" of HHV-6.
Some research has tied the less common "A variant" to chronic fatigue syndrome and multiple sclerosis (MS). This variant is difficult to find because it is neurotropic, meaning it prefers to live in brain tissue. Neurotropic infection doesn't show up in standard blood serum tests. It can be detected with a spinal tap.