Virology is the study of viruses and virus-like agents, including their taxonomy, disease-producing properties, cultivation and genetics. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. It is a subfield of microbiology or of medicine. Virus classification is a major branch of virology. According to the type of nucleic acid they use as genetic material virus is classified as DNA viruses, RNA viruses and reverse transcribing viruses.