Researchers discovered two new viruses infecting a plant-damaging organism called Phytophthora cactorum. When these viruses are present, the organism grows more slowly but becomes more aggressive at infecting plants. The viruses attach to special membranes inside cells and depend on cholesterol-like compounds for their replication. Scientists were able to remove the viruses from infected strains using specialized techniques, allowing them to compare infected and virus-free versions of the organism.