Veterinary virology is the study of viruses causing diseases and disorders in animals. It is an important branch of veterinary medicine. A fully infective virus is called a viron. The genomes of viruses are smaller than those found in prokaryotic and eukaryotic cells and in most viruses the nucleic acid is present in a single molecule. The genomes of viruses are of two types: single and double-stranded and viruses can contain either RNA or DNA. Due to these viruses are of three main groups based on their nucleic acid composition: DNA viruses, RNA viruses and viruses.