Viruses are microscopic particles that contain genetic material but are not living organisms. They invade host cells to reproduce and are made up of a protein coat that surrounds a core of nucleic acid. To reproduce, viruses must recognize, attach to, and inject their genetic material into a living host cell, where they assemble new viral particles that cause the host cell to burst and release the new viruses. There are two main types of viral reproduction: lytic viruses immediately replicate and kill the host cell, while lysogenic viruses integrate their DNA into the host's genome and remain dormant until activating to produce new viruses.