MicroRNAs MicroRNAs are recently discovered small endogenous non-coding RNAs. These small RNAs of ~22 nucleotide length are crucial post-transcriptional regulators of gene expression in a wide spectrum of normal and abnormal biological processes including antiviral defence, oncogenesis and development in higher eukaryotes. Recently several virus genomes have also been found to encode microRNAs. The present understanding of the biological functions of virus-encoded microRNAs remains sketchy, with evidence mainly derived from studies on individual or a small set of microRNAs encoded by the viruses and their cognate hosts. Survival strategies of the virus and counter strategies of host cells through miRNAs of host and viral origin and their respective targets form the crux of host virus interactions mediated by microRNAs. Thus microRNAs form a complex link between the regulatory networks of the host and the pathogen. A thorough understanding of the microRNA-mediated host-pathogen interaction is essential in understanding the basic pathophysiological changes associated with viral infections