A virus is a small infectious agent that replicates within living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Today, around 5,000 viruses have been reported in detail, although there are potentially millions. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. The study of viruses is known as virology, a sub-speciality of microbiology. Virology has two volumes. Volume 1 mainly discusses the latest research related to Human Immunodeficiency Virus (HIV).