This document provides an overview of viruses and virology. It discusses the general properties of viruses, including their structure, genomes, hosts, and methods of quantification. It describes the nature of the virion and details of viral structure. It also examines the virus-host relationship and viral replication cycle. Additionally, it covers viral diversity, including bacteriophages, animal viruses, retroviruses, and defective viruses. Finally, it discusses subviral entities such as viroids and prions.