Viruses and Simple Infectious Agents
19.1 What Is a Virus
1. A virus is a nucleic acid in a protein coat and perhaps an envelope. It must be in a cell to reproduce.
2. Viruses were discovered when scientists passed fluid from infected organisms through filters that held back bacteria, and then demonstrated that the strained material was infectious.
3. A virus’s protein coat is a capsid; the viral particle outside a cell is a virion. Capsids have a variety of shapes.
4. A viral envelope includes virally encoded proteins and lipids, some of which may have come from host cell membranes.
19.2 How Do Viruses Infect Cells and Reproduce?
5. A virus binds to a surface molecule on a host cell, then injects its nucleic acid or is engulfed. Insects assist viruses in infecting plant cells. The viruses then spread via plasmodesmata that link plant cells.
6. In a lytic infection, progeny virions are manufactured, assembled, and released. In a lysogenic infection, the bacteriophage nucleic acid integrates into the host DNA and remains hidden. Some animal viruses can exist hidden within a host cell in a latent infection.
7. The steps of a viral replication are determined by whether the genetic material is RNA or DNA, and whether it is single-stranded or double-stranded.
19.3 How Do Viruses Cause Disease?
8. Viruses cause disease by harming certain cell types, and indirectly by stimulating immune system responses.
9. The types of species that a virus infects are its host range. A virus may be carried in a reservoir species, in which it doesn't cause symptoms.
10. A zoonosis is an infection that humans acquire from another type of animal. Birds and bats spread some zoonoses.
11. Protection against viral infection includes restriction endonucleases in bacteria and immune systems in animals.
19.4 Other Infectious Agents
12. Viroids are infectious RNAs that affect plants.
13. A prion is a protein that can assume different conformations, some of which can convert other prions to the scrapie form and thereby cause transmittable spongiform encephalopathies.