So I stumbled across this article
http://vaccinenewsdaily.com/medical_countermeasures/330580-flavivirus-discovery-may-lead-to-new-treatments-vaccines/Since one of the papers was publshed in eLife it's open acess, so it's here (bit technical in places, but nice diagrams)
http://elifesciences.org/content/3/e01892Essentially, the RNA of some flaviviruses (such as Dengue virus and West Nile virus) use a host enzyme which breaks down the RNA (called an exonuclease, specifically Xrn1 in this case) to produce very small bits of RNA which causes symptoms to develop.
What they've shown in this paper, is that the non-degraded RNA have completely new folded structures which prevent the enzyme from breaking them down. This is interesting as these sorts of folds can be induced, meaning it's a potential new theraputic target (not just a cool little virus trick)!