It's actually the outbreaks which have got me thinking it's got to be endogenous. You have cluster outbreaks such as in Mella & Fluge's city, after people got a giardia infection, you have outbreaks after EBV and sometimes several people get it after the a harsh flu virus. When we know that it can break out given such triggers, we also know the virus must often lie latent, and burst out when the immune system is temporarily supressed somehow (triggers). What do you mean with your latter paragraph? I haven't heard of any such research.