HISTORY OF VIROLOGY:-
The historic reason for the discovery and characterization of viruses, and a continuing major reason for their detailed study, involves the desire to understand and control the diseases and attending degrees of economic and individual distress caused by them. As studies progressed, it became clear that there were many other important reasons for the study of viruses and their replication. Since viruses are parasitic on the molecular processes of gene expression and its regulation in the host cell, an understanding of viral genomes and virus replication provides basic information concerning cellular processes in general.
The whole development of molecular biology and molecular genetics is largely based on the deliberate choice of some insightful pioneers of “pure” biological research to study the replication and genetics of viruses that replicate in bacteria: the bacteriophages. (Such researchers include Max Delbrück, Salvadore Luria, Joshua Lederberg, Gunther Stent, Seymour Benzer, Andre Lwoff, François Jacob, Jacques Monod, and many others.)
The bacterial viruses (bacteriophage) were discovered through their ability to destroy human enteric bacteria such as Escherichia coli, but they had no clear relevance to human disease. It is only in retrospect that the grand unity of biological processes from the most simple to the most complex can be seen as mirrored in replication of viruses and the cells they infect.
The biological insights offered by the study of viruses have led to important developments in biomedical technology and promise to lead to even more dramatic developments and tools. For example, when infecting an individual, viruses target specific tissues. The resulting specific symptoms, as already noted, define their pathogenicity. The normal human, like all vertebrates, can mount a defined and profound response to virus infections. This response often leads to partial or complete immunity to reinfection. The study of these processes was instrumental to gaining an increasingly clear understanding of the immune response and the precise molecular nature of cell–cell signaling pathways. It also provided therapeutic and preventive strategies against specific virus-caused disease. The study of virology has and will continue to provide strategies for the palliative treatment of metabolic and genetic diseases not only in humans, but also in other economically and aesthetically important animal and plant populations.
Examples of the impact of viral disease on human history:-
There is archeological evidence in Egyptian mummies and medical texts of readily identifiable viral infections, including genital papillomas (warts) and poliomyelitis. There are also somewhat imperfect historical records of viral disease affecting human populations in classical and medieval times. While the recent campaign to eradicate smallpox has been successful and it no longer exists in the human population (owing to the effectiveness of vaccines against it, the genetic stability of the virus, and a well-orchestrated political and social effort to carry out the eradication), the disease periodically wreaked havoc and had profound effects on human history over thousands of years. Smallpox epidemics during the Middle Ages and later in Europe resulted in significant population losses as well as major changes in the economic, religious, political, and social life of individuals. Although the effectiveness of vaccination strategies gradually led to decline of the disease in Europe and North America, smallpox continued to cause massive mortality and disruption in other parts of the world until after World War II.
Despite its being eradicated from the environment, the attack of September 11, 2001 on the World Trade Center in New York has lead some government officials to be concerned that the high virulence of the virus and its mode of spread might make it an attractive agent for bioterrorism.
Other virus-mediated epidemics had equally major roles in human history. Much of the social, economic, and political chaos in native populations resulting from European conquests and expansion from the fifteenth through nineteenth centuries was mediated by introduction of infectious viral diseases such as measles. Significant fractions of the indigenous population of the western hemisphere died as a result of these diseases. Potential for major social and political disruption of everyday life continues to this day. As discussed in later chapters of this book, the “Spanish” influenza (H1N1) epidemic of 1918–19 killed tens of millions worldwide and, in conjunction with the effects of World War I, came very close to causing a major disruption of world civilization. Remarkable medical detective work using virus isolated from cadavers of victims of this disease frozen in Alaskan permafrost has lead to recovery of the complete genomic sequence of the virus and reconstruction of the virus itself (some of the methods used will be outlined in Part V). While we may never know all the factors that caused it to be so deadly, it is clear that the virus was derived from birds passing it directly to humans. Further, a number of viral proteins have a role in its virulence. Ominously, there is no reason why another strain of influenza could not arise with a similar or more devastating aftermath or sequela – indeed as of the spring/summer of 2005 there is legitimate cause for concern because a new strain of avian influenza (H5N1) has been transmitted to humans. At the present time, human transmission of H5N1 influenza has not been confirmed, but further adaptation of this new virus to humans could lead to its establishing itself as a major killer in the near future.
A number of infectious diseases could become established in the general population as a consequence of their becoming drug resistant, human disruption of natural ecosystems, or introduced as weapons of bioterrorism. As will be discussed in later chapters, a number of different viruses exhibiting different details of replication and spread could, potentially, be causative agents of such diseases. Animal and plant pathogens are other potential sources of disruptive viral infections. Sporadic outbreaks of viral disease in domestic animals, for example, vesicular stomatitis virus in cattle and avian influenza in chickens, result in significant economic and personal losses. Rabies in wild animal populations in the eastern United States has spread continually during the past half-century. The presence of this disease poses real threats to domestic animals and through them occasionally, to humans. An example of an agricultural infection leading to severe economic disruption is the growing spread of the Cadang-cadang viroid in coconut palms of the Philippine Islands and elsewhere in Oceania. The loss of coconut palms led to serious financial hardship in local populations.
Examples of the evolutionary impact of the virus–host interaction:-
There is ample genetic evidence that the interaction between viruses and their hosts had a measurable impact on evolution of the host. Viruses provide environmental stresses to which organisms evolve responses. Also, it is possible that the ability of viruses to acquire and move genes between organisms provides a mechanism of gene transfer between lineages. Development of the immune system, the cellular-based antiviral interferon (IFN) response, and many of the inflammatory and other responses that multicellular organisms can mount to ward off infection is the result of successful genetic adaptation to infection. More than this, virus infection may provide an important (and as yet underappreciated) basic mechanism to affect the evolutionary process in a direct way. There is good circumstantial evidence that the specific origin of placental mammals is the result of an ancestral species being infected with an immunosuppressive proto-retrovirus. It is suggested that this immunosuppression permitted an immunological accommodation in the mother to the development of a genetically distinct individual in the placenta during a prolonged period of gestation! Two current examples provide very strong evidence for the continued role of viruses in the evolution of animals and plants. Certain parasitic wasps lay their eggs in the caterpillars of other insects. As the wasp larvae develop, they devour the host, leaving the vital parts for last to ensure that the food supply stays fresh! Naturally, the host does not appreciate this attack and mounts an immune defense against the invader – especially at the earliest stages of the wasp’s embryonic development. The wasps uninfected with a polydnavirus do not have a high success rate for their parasitism and their larvae are often destroyed. The case is different when the same species of wasp is infected with a polydnavirus that is then maintained as a persistent genetic passenger in the ovaries and egg cells of the wasps. The polydnavirus inserted into the caterpillar along with the wasp egg induces a systemic, immunosuppressive infection so that the caterpillar cannot eliminate the embryonic tissue at an early stage of development! The virus maintains itself by persisting in the ovaries of the developing female wasps.
A further example of a virus’s role in development of a symbiotic relationship between its host and another organism can be seen in replication of the Chlorella viruses. These viruses are found at concentrations as high as 4 × 104 infectious units/ml in freshwater throughout the United States, China, and probably elsewhere in the world. Such levels demonstrate that the virus is a very successful pathogen. Despite this success, the viruses can only infect free algae; they cannot infect the same algae when the algae exist semi-symbiotically with a species of paramecium. Thus, the algae cells that remain within their symbiotes are protected from infection, and it is a good guess that existence of the virus is a strong selective pressure toward establishing or stabilizing the symbiotic relationship.
