ABSTRACT
All viruses depend for their existence on their ability to infect cells, causing them to make more virus particles. The infected cells generally die in the process. Even though the infection cycles of different types of viruses vary in detail, they all follow the same basic pattern. First, the viruses deliver their nucleic acid genome into the host cell, often in association with viral proteins; the mechanism of delivery varies depending on the virus, but the end result is the same. Second, the host cell’s biosynthetic machinery is subverted for the replication, transcription, and translation of the viral genes at the expense of cellular gene expression. Finally, progeny virus particles assemble in the infected cell and by one route or another leave it to infect a fresh host cell. Individual viruses can only infect a restricted range of hosts, and the host organisms usually have defense mechanisms, ranging from restriction enzymes in bacteria to the immune system of vertebrates. The combination of restricted host ranges and host defense mechanisms keeps in check the extent of viral infections.
