Recent research suggests that antiretroviral drugs designed to prevent HIV-positive individuals developing AIDS also have an effect on multiple sclerosis (MS). A study of 21 000 HIV-positive individuals treated at English hospitals between 1999 and 2011 found that they were 62% less likely to develop MS compared to non-HIV-positive individuals. One explanation for this could be that HIV infection actually combats the onset of MS in some way. However, some theories on the development of MS suggest that it has its origins in a viral infection.