I read in a 2009 review that
While mammals and C. elegans each have a single Dicer that makes both miRNAs and siRNAs, Drosophila has two Dicers: Dcr-1 makes miRNAs, whereas Dcr-2 is specialized for siRNA production. The fly RNAi pathway defends against viral infection, and Dicer specialization may reduce competition between pre-miRNAs and viral dsRNAs for Dicer. Alternatively, Dcr-2 and Ago2 specialization might reflect the evolutionary pressure on the siRNA pathway to counter rapidly evolving viral strategies to escape RNAi. In fact, dcr-2 and ago2 are among the most rapidly evolving Drosophila genes. C. elegans may achieve similar specialization with a single Dicer by using the double-stranded RNA-binding protein, RDE-4, as the gatekeeper for entry into the RNAi pathway. However, no natural virus infection has been documented in C. elegans. In contrast, mammals may not use the RNAi pathway to respond to viral infection, having evolved an elaborate, protein-based immune system.
Is it still true that no know natural virus infects C. elegans? And is this [thought to be] due to the effectiveness of its viRNA (virus-derived small interfering RNAs) immunity/defense mechanism?