This mini-review summarizes current knowledge of MSRV (multiple sclerosis-associated retrovirus), founder member of the type W family of human endogenous retroviruses (HERVs), its pathogenic potential and association with diseases. As retrotransposable elements, HERVs behave differently from stable genes, and cannot be studied with "Mendelian genetics" concepts only. They also display complex interactions with other HERV families, and with classical viruses. These concepts may contribute to unravelling the etiopathogenesis of complex diseases such as multiple sclerosis, schizophrenia, and other chronic multifactorial diseases.