at Indiana University, Bloomington

Brief History of Virology

Viruses are still a major cause of most human diseases.
We will begin of with a few examples of common viruses. One should note that viruses affect every " living" creature including bacterium,

protozoa,and yeast.

Animal viruses Plant viruses Other
Rabies Tobacco Mosaic bacterophage lambda
Smallpox cucumber mosaic T-even phages
Polio Brome Mosaic yeast viruses
hepatitis A,B,C    
yellow Fever   Scrapie ( prion)
Herpes   Mad Cow Disease ( prion)
Foor and Mouth Disease   Plant Viroids
AIDS ( HIV)   Hepatitis Delta.
Human T-cell leukemia    


Note: Some of these viruses such as kuru are "slow-viruses," and are models for degenerative diseases: These are caused by prions. Alzheimer's disease may be of a similar origin. .Diabetes,and rheumatoid arthritis may be viral related.This is quite controversial.
The majority of viral infections occur without any symptoms, they are subclinical. There may be virus replication without symptoms. In other cases virus replication always leads to disease, e.g. measles.
Some viruses may cause more than one type of disease state,e.g. measles, chicken pox. in other cases same symptoms may result from different virus infections ( hepatitis )..

Basic characteristics of Viruses

1-Although viruses are very heterogeneous, there is a unity of structure, basically protein and nucleic acid.Structure unique.
2-Replication similar.Note not binary division, but burst of virus particles.
3-Size: viruses are "filterable" agents.
4-Obligate (genetic) parasites--dependent on host cell genetic material.
5-Virus genome is either DNA or RNA not both.

Viruses have probably been around as long as life has existed.

In human history there are references to viruses in Homer. He writes of " rabid dogs". Also rabid dogs were known in Mesopotamia. In drawings from ancient Egypt, etc.drawings of people with withered legs, which could have been the result of polio. Smallpox probably played an important role in history of S. and Central America. Yellow fever was endemic, " Flying Dutchman" may have been due to this cause.
In looking at the economic history of the tulip trade in Hlooand, valuable tulips were variagated, which was the result of virus infection

Experimental virology really begins with the experiments of Jenner in 1798.The idea of vaccinationwas not truly novel It had been practiced by the Turks and possibly also in China. See paper by Langer, Immunization against Smallpox before Jenner in Scientific American reprints. Jenner did not know causative agent or reason for immunity, but noted that individuals exposed to cow pox did not suffer from small pox. Probably other peoples such as the Chinese were also aware of the technique of scarification. Known as variolation.

It is of interest that the virus we use today in the vaccination against small pox, is not the same as that used by Jenner. Sequence analysis indicates that it is not cow-pox, but a variant that arose during the last few hundred years. It is speculated that it may be of horse origin.

Perhaps the most important paradigm for microbiology was proposed by Koch and Henle, although known as Koch's postulates about this time. There was great difficult in applying these Koch's postulates to viral disease.This postulate requires a stop of in vitro growth, which is not always possible with animal viruses. Indeed even today we have had trouble applying these postulates to HIV and the AIDS epidemic.

Koch's postulates: Definition of a pathogen

1. The organism must regularly be found in the lesions of the disease.
2. The organism must be isolated from diseased host and grown in pure culture
3. The inoculation of such a pure organism into a host should initiate the disease
4. The organism must be recovered once again from the organism.

Two other important landmarks were the proving by Pasteur that spontaneous generation of organisms did not occur ( see figure1.4 in Flint et al), and the development of limiting dilution by Lister.
It was also Lister who developed the concept of sterility to obtain pure cultures.

1881-1885, Pasteur. Use of animals as model for growing and studying viruses. Passage of rabies virus through the brain of rabbits, produced an attenuated vaccine. When infected into dogs produced mild infection, with increased latent period. However Pasteur did not try to identify the infectious agent.

This method of attenuation, by producing attenuated virus in tissue culture still used today. Importance of Pasteur's work was not only the characterization of rabies, but also the whole concept of using animals to develop model systems for the study of viruses.

Discovery Period. 1886-1903.
Plant virology

Knowledge of variegation in tulips dates from 16th century. 1886 Mayer (in Wegeningen, Holland) demonstrated infectivity of TMV. One could get disease spread from tobacco pulp or sap. However Koch's postulates could not be satisfied.

Ivanowski observed/looked for bacteria like substance. 1898, Beijerink demonstrated filterable characteristic of the virus and thinks virus was obligate parasite. Chamberlain filter-candles of unglazed porcelain had been invented for water purification. These filters retained bacterium, and had a pore size of 0.1-0.5 micron. Filters were made of diatomaceous earth (clay)-kieselguhr. Also substance could pass through a layer of agar--contagium vivum fluidum "contagious living liquid." Boiling, or treating with formaldehyde abolished infection. The term for such agents was coined--virus, Latin from Greek, toxin. See description of different types of filters in Flint et al. fig 1.4.

In the 1930's Elford--"collodion" membranes, and could show that viruses are particulate and of discrete size (10 nm for FMDV)

10-9 meter = 1 n meter. Range of size.

Animal virology
Loeffler and Frosch (1898) reported that the infectious agent of foot and mouth diseases virus was a filterable agent. This could have been a toxin, however since on dilution one still obtained material, sugget that it was a replicating agent.

In 1900 first human disease shown to be caused by a filterable agent was Yellow Fever by Walter Reed. He established that yellow fever virus was present in the blood of patients during the febrile period, and that the virus could be transmitted by mosquitoes. Yellow Fever had a very high mortality rate. In 1853 there was an epidemic in New Orleans with mortality of 28%.
Infectivity controlled by destroying mosquito population.

There were many human volunteers in the study of yellow fever, with many deaths. Thus the basic concept arose about this time of a filterable agent too small to be observed in light microscope but able to cause disease by multiplying in living cell.

1908 Ellerman and Bang demonstrated that certain types of tumors (leukemia of chicken) were infectious, and that the infectious material had characteristics of cell free agent.

1911 Peyton Rous discovered that non-cellular agent could transmit solid tumors: virus now known as the Rous Sarcoma virus (RSV).

Bacteriophage Era
Twort (1915) working with vaccinia virus, attempting to grow the virus in medium, got bacterial contamination, micrococcus noted the existence of a substance that cleared bacterial cultures, termed this substance bacteriophage. Twort abandoned this work after World War I and reported it in a short paper

1917 D'Herelle, a Canadian working at the Pasteur made same discovery with the Dysentery bacillus. Developed limiting dilutions, plaque assays, and proposed that virus was particulate in nature.
Note: Novel by Sinclair Lewis: Arrowsmith; idea was to use bacteriophage to destroy bacterial infections. This idea constantly re-occurs.

Plant virology
Chemists had shown that virus could be purified using all the techniques of protein chemistry.

1935: Wendell Stanley, an organic chemist, reported the crystallization of T.M.V. This material was infectious.

Analysis of the crystals showed them to be predominantly proteins in nature of very high molecular weight (- 90% protein). The observation that viruses could be crystallized gave rise to controversy of whether TMV was a living organism, or a "pure" inert chemical substance. 0.5% phosphorus and 5% RNA.

Stanley defined TMV as an "autocatalytic" protein, which may be assumed to require the presence of living cells for multiplication cf. with trypsinogen-trypsin.

1938-1970. Development of the phage group and Molecular Biology

The 1940's and 1950's was the era of the Bacteriophage and the animal virus. The great surge in virology occurred as a result of the work with bacteriophage in the 1940's by Delbruck and co-workers. Delbruck considered father of modern molecular biology although really was a physicist interested in genetic structure.

About the same time identification or correlation was found between tumor viruses and tumors in mice.

Delbruck and Ellis (1940) worked out the replication cycle of the bacteriophage using the one step growth curve experiment, and in 1952 Hershey and Chase showed that it was the nucleic acid portion that was responsible for the infectivity and carried the genetic material.

Delbruck worked with Luria to form the phage group. The first phage CSH meeting took place in 1947 with 8 participants. From this meeting and group developed the whole field of molecular biology.

In 1954 Watson and Crick elucidated the structure of DNA.

Discovery of lysogeny by Lwoff in 1949, and later zygotic induction was an important milestone in tumor biology.

1.It showed that virus could behave like a bacterial gene on the chromosome.
2. Showed quiescent negative regulation
3. Led to Jacob/Monod model of gene induction and repression, the foundation of the operon model.

The development of lambda genetics is one of the great intellectual adventures of our time. Work on lysogeny led to the idea that genetic material (foreign and host) could be transferred by viruses.

See table 1.1 from Flint et al.

Animal Virology and tissue -Culture

Simultaneously with this work there was the development of tissue culture techniques. Alex Carel in 1910 had already shown that it is possible to grow chick embryo fibroblasts in culture, and in the 1930's chick embryos were using for growing viruses such as pox virus. By 1940 Earle and associates had developed media for the growth of cell lines.

1949 Enders et al. worked out human tissue culture--used for growth of poliovirus. In 1954 received Nobel Prize. Era of polio research extremely important in virology led to quantitative methods of measuring animal viruses--development of defined media for tissue culture, and eventually animal virus plaque assays.

1953 Salk vaccine (killed poliovirus) introduced and by 1955 poliovirus had been crystallized. Later Sabin introduced attenuated polio vaccine. Dulbecco in 1950's described animal virus plaque assays similar to phage plaque assays, and viral transformation analogous to tumor formation in vitro.

1965 Spiegleman and coworkers obtained complete replication of nucleic acid of QB phage in a cell free system; thus showing the RNA was also genetic material.

1970 Temin and Baltimore independently demonstrated the presence of reverse transcriptase in RNA tumor viruses (RNA --> DNA --> RNA).

1980's ....Cloning of viral genes.
..................Sequencing of complete genomes
..................Production of hybridomas.
..................Isolation of AIDS virus.
..................HTLV series.
..................Oncogene characterization and relationship of virus to cancer.
................. Monoclonal antibodies.
................ .Discovery of cytokines and growth factors.
................ .Knowledge of immunology.
................ .Development of PCR techniques.

Gene therapy a product of virus, with the development of vector systems. By 1977 we have the complete eradication of smallpox. Last natural case. What should we do with smallpox in labs? we wil discuss the threat of bioterrosim at a later time.

Definition of a Virus

Lwoff in 1957 proposed to define a virus as potentially pathogenic entities with an infectious phase and: (1) possessing only one type of nucleic acid, (2) multiplying in the form of their genetic material (3) unable to undergo binary fission and (4) devoid of a Lipmann system. This is basically a negative definition, and stresses the non-cellular nature of viruses.

Viruses are infectious entities whose genomes are either RNA or DNA, and that replicate inside living cells using the cellular synthetic machinery and causing the synthesis of specialized components that can transfer the viral genome to other cells.

Thus viruses are intracellular parasites at the genetic level. They are obligatory parasites--cannot be grown outside of the cell. Absolutely dependent on host cell energy yielding and protein synthetic apparatus.

Viruses persist in two stages:

1) dormant phase--extracellular; this phase is neither "alive nor dead" rather should be described as functionally active or inactive.

2) Vegetative phase--intracellular.

One should note that the nucleic acid of the virus, in some cases, is infectious.

All viruses consist of RNA or DNA and a protein coat. Some viruses are enclosed within envelopes that contain both proteins and lipid.

An interesting question: Is a virus an organism? Is it more of an organism than a chromosome? Could viruses have evolved from chromosomes or some other organelle? What about transposons?