This lecture covers the structure, function, and replication of viruses. Viruses are submicroscopic infectious agents that replicate inside living cells, infecting all types of life forms. They are nature's nanotechnology, capable of invading cells to reproduce. The lecture also discusses the diversity of viruses, their sizes, and the viral replication process using host cell machinery. Additionally, it explores the interaction of bacteriophages with bacteria, viral populations' growth, and the immune system's response to viral invasion.