Viruses Viruses are microscopic infectious agents that require a host cell to replicate. They infect all forms of life, including animals, plants, fungi, and bacteria, causing a wide range of diseases. Characteristics of Viruses Structure of Viruses Classification of Viruses 1. Based on Genetic Material 2. Based on Host 3. Based on Mode of Transmission Viral Replication Viruses follow a multi-step process to replicate within host cells: Diseases Caused by