Nidovirales is an order of viruses known for their unique genetic structure and replication process. This order includes several families, such as Coronaviridae, which contains viruses responsible for diseases like COVID-19, and Arteriviridae, which primarily infects animals. Nidovirales viruses have a positive-sense single-stranded RNA genome. These viruses are characterized by their ability to produce a nested set of subgenomic RNAs during replication. This feature allows them to efficiently express multiple proteins from a single RNA molecule. Understanding Nidovirales is crucial for developing vaccines and treatments for the diseases they cause.