"...Thus, we cannot reject the assumption that the effect of the filtered lymph is not due to toxicity, but rather to the ability of the agent to replicate...."
Freidrich Loeffler
1898
Definition of a Virus
- Submicroscopic (nm)
- Viruses are collections of genetic information directed toward one end: their own replication
- They are the ultimate and prototypical example of "selfish genes".
- The viral genome contains the "blueprints" for virus replication enciphered in the genetic code, and must be decoded by the molecular machinery of the cell that it infects to gain this end.
- They thus are obligated parasites dependent on the metabolic and genetic functions of living cells.
- Have evolved and ensured propagation and replication in free-living organisms that are between 10 and 10,000,000 times their size and genetic complexity.
What Are Viruses?
- An infectious particle incapable of replicating outside of the host cell, consiting of an RNA or DNA genome surrounded by a protein coat (capsid) and, for some viruses, a membranous envelope.
- The have no independent metabolic activity of their own but depend upon a host cell for their metabolites and organelles required to express and replicate their genomes, and to generate viruses.
- Viruses are intracellular parasites that require the metabolic activities of a host to support their growth.
A Challenge to the Definition of Viruses
- E. Wimmer, A. Molla, and A. Paul (1991) announced that they successfully grew entire polioviruses in test tubes containing ground up human cells, but not live cells.
- RNA from polioviruses was added to the cell-free extract, and about 5 hours later complete new viral particles began to appear.
- This has not yet been duplicated with other viruses.
Similarities of Viruses and Bacteria
- Some bacteria like mycoplasma are also small enough to pass through the filters used for bacterial sterilization.
- Rickettsia and Chlamydia are also intracellular parasites and require special metabolites from their host cells for growth.
Virology provides the basis for much of our most fundamental understanding of modern biology, genetics, agriculture and medicine.
Virology has Impacted the Study of:
- Macromolecules
- Processes of cellular gene expression
- Mechanisms for generating genetic diversity
- Processes involved in the control of cell growth and development
- Aspects of molecular evolution
- The mechanisms of disease and host response
- The spread of disease in populations
Virus in Human History
The Pre-historical Record
- The earth is estimated to be about 4.5 billion years old.
- Fossil evidence for bacteria suggests that the planet has supported life for at least 3.5 billion years.
- Amoebas and protozoans probably emerged about 1.5 billion years ago.
- Homo sapiens appeared on earth about 100,000 years ago.
Historical Perspectives
- Viruses have been know as distinct biological entities for little more than a century.
- Evidence can be found among the earlier recordings of human activity.
- Methods for combating viral disease were practiced long before the first virus was recognized.
- We can infer from scattered glimpses of ancient history that viruses have long been a part of human experience.
The Most Ancient Evidence of Viral Disease of Humans is Much More Recent
- A bas-relief of an Egyptian priest of 1,500 yrs BC shows a shriveled leg (polio).
- Although evidence for what may have been smallpox occurs in Egyptian mummies and in ancient Chinese documents, neither smallpox or measles seem to have been known to Hippocrates (460-377 BC). Mumps and influenza were recorded in the island of Thasos during Hyppocrates time.
- Apparently smallpox and measles appeared in China (37 AD).
- These two diseases also contributed to the decline of the Roman Empire.
- Smallpox and measles were the principal cause of the sudden demise of the Aztec and Inca empires.