Disturbed cell-to-cell communication has turned up repeatedly as a problem in diseases that are hard to pin down, including those described as psychological. We now learn that a cellular protein called Arc has a striking resemblance to a viral capsid, including the ability to carry RNA/DNA. This same protein was missing in knockout mice with defective memory. At this point we don't know exactly what this is doing, or if it is actually carrying RNA messages between cells. However, this really looks like a possible link between viral diseases and cognitive function. An immune response against a virus could also impact a protein important for mental function. From an evolutionary perspective this is a tantalizing clue that viruses may have been responsible for horizontal transfer of useful genetic information between species, although that is quite speculative at this point. The virus could have started out as a pure pathogen, but then a protein introduced by viruses has been coopted into performing a useful function. Evolution is supremely opportunistic.