Is multiple sclerosis a sexually transmitted infection?

It is proposed that multiple sclerosis may be transmitted chiefly by sexual contact. Arguments favouring this include: migration studies that suggest a transmissible agent in adolescence; clusters of multiple sclerosis which have occurred in low prevalence areas following entry of young males; the similarity of multiple sclerosis to tropical spastic paraplegia, a known sexually transmitted infection with resemblance to primary progressive multiple sclerosis; an increased rate in drug misusers; a similar age of onset and sex pattern to that found in sexually transmitted disease; increased incidence of multiple sclerosis in those using oral contraceptives; low multiple sclerosis rates in societies with a strict moral code; longitudinal shifts in sex prevalence that show an increase in women after the sexual revolution of the 1960s; and important exceptions to the worldwide distribution corresponding to countries with permissive attitudes to sex. Family, conjugal pair, twin, and adoption studies are compatible with an infectious cause of multiple sclerosis if this is sexually transmitted. It is not proposed that sexual transmission is the only cause but that inherited factors create a susceptibility to a sexually transmitted neurotropic agent. It is hoped this hypothesis might encourage a new direction of neurological research.