I think alex3619 probably answers that question: This is my thinking (pending further info): Sudden onset First you get an XMRV infection, maybe you have it for years with few if any symptoms, like HIV patients. Sudden onset patients then get a nasty virus that speeds up the XMRV-->XAND progression. HHV-6 is a known progression factor for HIV-->AIDS, for example. Gradual onset You get an XMRV infection which gradually damages your immune system over time. Eventually your immune system is so damaged that you have XAND. Also similar to a path in HIV-->AIDS Gradual-sudden onset This may occur when the virulent 2nd virus hits early in XMRV infection so there's less XMRV to spread (or better immune system function), or perhaps the 2nd virus is not as serious (typical flu) but comes later in the XMRV infection. XMRV is not HIV. It is different in a number of important ways. But there are some similarities that may be helpful in thinking about XMRV and XAND. It looks to me like there's plenty of room in the XMRV hypothesis for all the kinds of onset we've seen. It's only when we assume all our symptoms are due directly to XMRV or that XMRV acts like the viruses we're used to (flu, mono, chicken pox) that we run into trouble. It seems most likely to me that our symptoms are primarily due to secondary infections, which themselves are the result of immune deficiencies caused directly by XMRV. CNS infections, in particular, seem very likely. Several of the herpes viruses can spread to the CNS, especially when not fully suppressed by the immune system.