This disease has been recognised for about two decades in the UK, but is becoming progressively more serious. It is also known from mainland Europe and North America. A virus called Helleborus net necrosis virus (HeNNV) is associated with the disease and believed to be the cause. The virus is thought to be transmitted by the hellebore aphid, Macrosiphum hellebori. However, conclusive evidence for both suspicions is still lacking.
Aphids transmit viruses by feeding on the sap of plants with virus infection, and thus contaminating their mouthparts with virus particles. When they fly to healthy plants and begin to feed, they then infect the plant with virus. Seriously infected plants are so stunted they are unlikely to be attractive to aphids and the most dangerous plants are probably those that are still lightly infected and suitable for aphid feeding.