Viruses may spark hydrocephalus by exploiting a surprising weakness of cells that circulate fluid in the brain, called ependymal cells, report scientists. The researchers discovered that ependymal cells from mice require a continuous production of a transcription factor called Foxj1 to maintain their shape and function. Viruses known to infect the brain have found a way to shut down the production of Foxj1 and disable the cells, the researchers show.