Long before scientists could deﬁne the nature of speciﬁc viruses, viral diseases—smallpox and rabies—had provided the most signiﬁcant and dramatic examples of the potential of preventive inoculations. Because the meaning of the Latin word virus has undergone many changes in two millennia of usage, the modern reader is likely to be con- fused upon ﬁnding the term in ancient texts. The ﬁrst and most general meaning of virus was slime, presumably unpleasant, but not necessarily dangerous. However, Latin authors increasingly used the term with the implication of poison or venom, something menacing to health, or a mysterious, unknown infectious agent. Thus, both the Roman writer Celsus (ca. 14-37) and Louis Pasteur could speak of the virus of rabies.
Medieval scholars generally used virus as a synonym for poison. In
medical treatises of the sixteenth and seventeenth centuries, translators usually replaced the Latin term virus with the English word venom. Seventeenth-century writers referred to a virus pestiferum or virus pestilens in discussing infectious diseases. Eighteenth-century medical writers applied the term virus to the contagion that transmitted an infectious disease, as in Edward Jenner’s discussion of the ‘‘cow-pox
virus’’ in the pustular lymph that transmitted the disease. For medical writers in the early nineteenth century, virus stood for the obscure causative principles of infectious diseases. The vagueness of the term made it particularly attractive.
After the establishment of germ theory in the late nineteenth century, virus was used in the general sense of ‘‘an agent with infectious properties.’’ When submicroscopic ﬁlterable infectious agents were discovered in the 1890s, the term virus was applied to these mysterious entities. Even if the causative agent for an infectious disease had not been identiﬁed, Pasteur argued that ‘‘Every virus is a microbe.’’ Specu- lating about the still unknown causes of various infectious diseases, Koch suggested that pathogenic organisms different from bacteria might be discovered. The exceptional agents that were known at the time were larger than bacteria, most notably the protozoan that causes malaria, but there was no theoretical reason to rule out the existence of smaller parasites. Among microbiologists, however, respect for Koch’s postulates might have inhibited virology, as well as protozoology, because it was virtually impossible to culture such entities in artiﬁcial media. Koch himself did not let bacteriological dogma inhibit his work on tropical medicine even where the microbes could not be cultured in the laboratory.
By the end of the nineteenth century, the techniques of micro- biology were sufﬁciently advanced for scientists to state, with a high degree of conﬁdence that certain diseases were caused by speciﬁc bac- teria or protozoa. However, the infectious agents of some diseases refused to be isolated by conventional techniques. Eventually, exotic, but visible, pathogens (rickettsias, chlamydias, mycoplasmas, and bru- cellas) joined the classical fungi, bacteria, and protozoa. Because some of the exotic pathogens had complicated life cycles and were difﬁcult to culture in vitro, it seemed possible that members of these groups might be the undiscovered agents of various infectious diseases.
Therefore, in the early twentieth century, the term virus was gener-
ally restricted to the class of ‘‘ﬁlterable-invisible’’ microbes. Such microbes were deﬁned operationally in terms of their ability to pass through ﬁlters that trapped bacteria and their ability to remain invisible to the light microscope. The criterion of ﬁlterability was the outcome of work conducted by Pasteur’s associate Charles Chamberland (1851–1908), who discovered that a porous porcelain vase could be used to separate visible microorganisms from their culture medium. This technique could be used in the laboratory to prepare bacteria-free liquids and in the home to prepare pure drinking water. Chamberland was also instrumental in the development of the autoclave, a device for sterilizing materials by means of steam heat under pressure. However, technique-based criteria provided little insight into the genetic and biochemical nature of viruses. As scientists closed in on
the invisible-ﬁlterable-viruses, they discovered that their operational criteria were not necessarily linked. The failure of infectious agents to grow in vitro was not a satisfactory criterion either, because scientists could not exclude the possibility that exotic microbes might need special media and growth conditions. A more radical explanation for the failure to identify the causative agents of some apparently infectious diseases was that some microbes might be obligate parasites of living organisms that could not be cultured in vitro on any cell-free culture medium.
Although for the sake of human health, it would have been better if viruses had totally destroyed all tobacco plants, progress in virology owes a great deal to this pernicious product of the New World, because it was the tobacco mosaic virus (TMV) that established Adolf Eduard Mayer, Martinus Beijerinck, and Dimitri Ivanovski as the founders of virology. The study of plant virology can be traced to 1886 when Adolf Eduard Mayer (1843–1942) discovered that tobacco mosaic disease (TMD) could be transmitted to healthy plants by inoculating them with extracts of sap from the leaves of diseased plants. Unable to culture a tobacco mosaic disease microbe on artiﬁcial media, Mayer ﬁltered the sap and demonstrated that the ﬁltrate was still infectious. Mayer was certain that his microbe must be a very unusual bacterium. In 1892, Dimitri Iosifovitch Ivanovski (1864–1920) demonstrated that the infec- tious agent for tobacco mosaic disease could pass through the ﬁnest ﬁlters available, but all attempts to isolate or culture the ‘‘tobacco microbe’’ were failures.
Apparently unaware of Ivanovski’s work, Martinus Willem Beijer- inck (1851–1931) also reported that a ﬁlterable agent caused TMD. Thinking about the way in which a small quantity of ﬁltered plant sap transmitted the disease to a large series of plants, Beijerinck con- cluded that TMD must be caused by a contagium vivum ﬂuidum that could pass through ﬁlters and reproduce within the living plant tissues. On the basis of reports in the botanical literature, Beijerinck thought soluble germs could cause many other plant diseases.
Similar observations were made by Friedrich Loefﬂer (1862–1915)
and Paul Frosch (1860–1928) in their studies of foot-and-mouth disease (FMD), the ﬁrst example of a ﬁlterable virus disease of animals. Attempts to culture bacteria from lesions in the mouths and udders of sick animals were unsuccessful. Even after passage through a Chamberland ﬁlter, however, the apparently bacteria-free ﬂuid from FMD lesions could transmit the disease to cattle and pigs. Filtered ﬂuids from these ani- mals could transmit the disease to other experimental animals. Their experiments and calculations suggested that only a living agent, capable of reproducing itself could continue to cause the disease after passage through a series of animals. Loefﬂer and Frosch suggested that other infectious diseases, such as smallpox, cowpox, and cattle plague, might be caused by similar ﬁlterable microbes. Nevertheless, they continued
to think of the infectious agent as a very small and unusual microbe rather than a fundamentally different entity.
Scientists later demonstrated that FMD is a highly infectious, air-
borne viral disease that attacks cloven-hoofed livestock animals like cows, sheep, goats, and pigs. The FMD virus is a member of the picor- navirus family, which includes many important human pathogens, such as poliovirus, hepatitis A virus, and rhinovirus. Picornaviruses are char- acterized by a small RNA genome. FMD is generally not regarded as a threat to humans who consume meat or pasteurized milk from affected animals, but people in close contact with infected animals can acquire the disease. In the 1830s scientists apparently infected themselves with FMD by inoculation and by drinking milk from infected cows. Proven cases of FMD in humans have occurred in several countries in Europe, Africa, and South America. Nevertheless, human cases appear to be extremely rare, even when large numbers of farm animals are affected.
Foot-and-mouth disease was introduced into the Americas in 1870. The disease was soon reported in parts of the United States, Argentina, Chile, Uruguay, Brazil, Bolivia, Paraguay, and Peru. Known outbreaks of FMD occurred in the United States from the 1870s to the 1920s, from New England to California. In the 1950s, the disease was reported in Venezuela, Colombia, Ecuador, and Canada. A Pan-American Foot- and-Mouth Disease Center was established in 1951. Using information gathered from participating countries, the Center developed plans for FMD eradication. By 2000, Chile, Uruguay, Argentina, Paraguay, and parts of Brazil were declared free of FMD. Although many authori- ties assumed that FMD had been virtually eradicated from Western Europe, a major epidemic in 2001, the ﬁrst since 1967, costs millions of dollars and resulted in the destruction of more than 1 million animals in the United Kingdom alone. Vaccination is used in countries where the disease is still endemic, but because vaccinated animals test positive for FMD antibodies, countries where vaccination is practiced cannot call their livestock ‘‘disease-free’’ and they cannot export to other nations. British scientists think meat from animals with FMD was illegally brought into England and fed to pigs.
In 1915, Frederick William Twort (1877–1950) discovered that
even bacteria could fall victim to diseases caused by invisible viruses. As Jonathan Swift (1667–1745) had suggested in a satirical poem on the microscope, naturalists might use the instrument to prove that ﬂeas were preyed on by smaller ﬂeas that were, in turn, attacked by still smaller ﬂeas. While trying to grow viruses in artiﬁcial medium, Twort noted that colonies of certain bacteria growing on agar sometimes became glassy and transparent. If pure colonies of this micrococcus were touched by a tiny portion of material from the glassy colonies, they too became transparent. Like the infectious agent of many mysterious plant and animal diseases, these so-called Twort particles were ﬁlterable.
World War I interrupted Twort’s work on this problem and his paper had little immediate impact on microbiology. Twort later became obsessed with speculative work on the possibility that bacteria evolved from viruses that had developed from even more primitive forms.
While working on the dysentery bacillus at the Pasteur Institute, Fe´lix d’He´relle (1873–1949) also discovered the existence of bacterial viruses. In 1917, he published his observations on ‘‘An invisible microbe that is antagonistic to the dysentery bacillus.’’ He never acknowledged that Twort, who published two years earlier, had discovered the same phenomenon. Because the invisible microbe could not grow on labora- tory media or heat-killed bacilli, but grew well in a suspension of washed bacteria in a simple salt solution, d’He´relle concluded that the anti-dysentery microbe was an obligate bacteriophage, that is, an eater of bacteria. Bacterial viruses were sometimes called Twort–d’He´relle particles. The invisible microbe was found in stool samples of patients recovering from bacillary dysentery. When an active ﬁltrate was added to a culture of Shiga bacilli, bacterial growth soon ceased and bacterial death and lysis (dissolution) followed. A trace of the lysate produced the same effect on a fresh Shiga culture. More than 50 such transfers gave the same results, indicating that a living agent was responsible for bacterial lysis.
Speculating on the general implications of the phenomenon he had discovered, d’He´relle predicted that bacteriophages would be found for other pathogenic bacteria. Although the natural parasitism of the invis- ible microbe seemed species speciﬁc, d’He´relle believed that laboratory manipulations could transform bacteriophages into ‘‘microbes of immu- nity’’ with activity against human pathogens. d’He´relle suggested that phages were involved in natural recovery and the end of epidemics. American novelist Sinclair Lewis (1885–1951), in collaboration with medical writer and microbiologist Paul de Kruif (1890–1971), explored this idea in Arrowsmith (1925). Although experimental tests of ‘‘phage therapy’’ were generally abandoned when antibiotics appeared as
‘‘miracle drugs,’’ the method has been used in the former Soviet Union,
and certain traditional Indian cures may employ naturally occurring bacteriophages. In 1896, for example, a Western scientist reported that water from the Ganges River in India, traditionally known for its curative properties, was lethal to the cholera vibrio.
The hope that bacteriophages could be trained in the laboratory to serve as weapons in the war on bacteria was not realized in the twentieth century, but researchers continue to explore the possibility that viruses might be recruited to attack drug-resistant bacteria. An estimated
90,000 Americans died in 2000 of hospital-acquired infections caused by antibiotic-resistant bacteria. Some scientists think that phages that prey on the tubercle bacillus might provide useful insights into the microbe’s pathogenicity, as well as new methods of diagnosis and drug
screening. Studies of the genomes of phages that attack tubercle bacteria suggest genetic exchanges between phages and their bacterial hosts. Many researchers are skeptical about this approach, primarily because of possible adverse effects caused by introducing a self-replicating virus into a patient’s bloodstream. Some drug companies have, however, explored the use of genetic engineering to control potentially useful
‘‘therapeutic phages’’ that could be given orally or as topical treatments.
In order to ﬁnd a virus that could kill a speciﬁc bacterial pathogen, researchers had to use a mixture of viruses. Critics warn that phage preparations might be contaminated with unknown strains. Even if highly puriﬁed preparations are used, dangerous strains of viruses might arise through recombination or mutation. Moreover, replicating viruses might also acquire and express genes for toxins or learn how to attack the cells of the patient instead of the bacterial target. Some researchers hope that genetic engineering can be used to produce very speciﬁc viruses and, therefore, reduce the risks. Others argue that naturally occurring viruses—already engineered by Mother Nature—are likely to be superior to, as well as less costly to produce, than modiﬁed viruses. One approach is to use phages to kill Salmonella and Listeria, often associated with food poisoning, during food preparation.
In a practical, rather than philosophical sense, many arguments
about the nature of viruses faded from the picture as researchers in the 1930s and 1940s examined them with new biochemical techniques. By the 1940s, biochemists were discovering just how complicated bio- logical macromolecules could be. Advances in biochemistry supported the concept of the virus as a complex entity on the borderline between cells, genes, and molecules. Viruses could, therefore, be described as particles composed of a protein overcoat and an inner core of nucleic acid that is capable of entering a host cell and taking over its metabolic apparatus. As to just what viruses are and where they ﬁt into the scheme of things among plants and animals, microbes and macromole- cules, living and nonliving, French microbiologist Andre´ Lwoff’s (1902–
1994) paraphrase of a famous line by Gertrude Stein seems the most
appropriate answer: ‘‘Viruses should be considered viruses because viruses are viruses.’’
Stories about the Human Genome Project are commonly pub- lished in the popular press and newspapers. In contrast, the sequencing of microbial genomes generates little publicity. Microbial genomics may have many practical applications for better vaccines, safer fermented foods and beverages, biodefenses, cleaner environment, and better health. Although the complete genomes of some one hundred microbes were sequenced by 2003, scientists note that we really know very little about the microbial world.
Research on diseases attributed to slow viruses, viroids, and prions
suggests that other invisible, mysterious, and perhaps menacing creatures
may well exist in the submicroscopic world. Unlike viruses, viroids appear to be pathogens consisting of small, single-stranded RNA mole- cules without a protein coat. Between 1971, when Theodor O. Diener (1921–) discovered that the infectious agent responsible for potato spin- dle tuber disease was a novel pathogen consisting of naked RNA, and
2001, about 30 viroid species and hundreds of variants had been stud- ied. Viroid diseases affect many plants, from avocadoes to coconuts, but viroids may also be involved in tumor formation and other diseases of animals. Despite the excitement generated by studies of viroids and other small RNAs, many questions remain about how viroids replicate, move from cell to cell, and cause disease. Viroids have been called evo- lutionary fossils and relics of pre-cellular evolution, but their discovery has stimulated research into interaction between foreign RNA mole- cules and human diseases.
Viroids have been called ‘‘naked intruders,’’ but because they con-
tain nucleic acid, they still seemed to ﬁt into the fundamental framework, or Central Dogma, of molecular biology, that is, the ﬂow of genetic information from nucleic acids to proteins. Prions, the most bizarre of all the infectious agents discovered in the twentieth century, challenge the Central Dogma, as well as the idea that ‘‘viruses are viruses,’’ at least in the case of disorders that were originally attributed to ‘‘slow viruses.’’ In 1982, Stanley B. Prusiner (1942–) coined the term prion, which stood for ‘‘proteinaceous infectious particle.’’ The diseases attributed to prions are known as transmissible spongiform encephalopathies (TSE), that is, degenerative diseases of the central nervous system. Prion diseases of animals include scrapie in sheep and goats, transmissible mink encepha- lopathy, chronic wasting disease of mule deer and elk, feline spongiform encephalopathy, and bovine spongiform encephalopathy (BSE, com- monly known as mad cow disease). Human diseases attributed to prions include Creutzfeldt–Jakob Disease (CJD) and a new variant (vCJD) that appears to be related to BSE, Fatal Familial Insomnia, Gerstmann– Sra¨ u¨ ssler–Scheinker syndrome, and kuru.
The idea that some neurological degenerative diseases might be
caused by a novel infectious agent was suggested by Carleton Gajdusek’s (1923–) studies of kuru, a disease found only among the Fore people of New Guinea. Based on ﬁeld studies, Gajdusek came to the conclusion that kuru was transmitted by mourning rituals during which women and children handled and ate the brains of deceased relatives. After cannibalism was outlawed, the incidence of the disease decreased. Using brain tissue from victims of kuru, Gajdusek and his associates were able to transmit the disease to chimpanzees. Symptoms did not appear, how- ever, until about two years after inoculation. Laboratory experiments by Gajdusek and others suggested that kuru, scrapie, and CJD might be caused by similar infectious agents. Gajdusek, who was awarded a Nobel
Prize in 1976, thought that the infectious agent must be an unconven- tional ‘‘slow virus.’’
Many aspects of the history of kuru seem relevant to the still
unfolding story of BSE and vCJD. Scrapie, an old Scottish name for a disease of sheep and goats, has been known since the eighteenth century, but until the 1980s when BSE ﬁrst appeared in England, there was no evidence of transmission to cows or humans. Scientists believe the BSE epidemic began when a nutritional supplement containing the rendered remains of sheep and cows was added to cattle feed. By essentially transforming domesticated herbivores into carnivores, or even cannibals like the Fore victims of kuru, the new dietary regimen presumably created an unprecedented opportunity for scrapie agents to infect cattle. As the mad cow epidemic reached its peak in 1992, millions of cattle were destroyed, but by then contaminated meat products had probably entered the food chain. The World Health Organization warned in 2003 that many countries, particularly in Eastern Europe and Southeast Asia, were at risk for mad cow disease, even though the worst appeared to be over in Britain. Mad cow disease appeared in areas in Europe, Southeast Asia, Canada, North Africa, and the United States that used contaminated feed.
Not all prion infections can be transmitted from one species to
another, but the new variant of CJD, designated vCJD, has been attrib- uted to the consumption of beef from animals with BSE. Much about the transmission of prion diseases remains obscure, as demonstrated by the relatively small number of human cases that occurred in Great Britain, compared to the millions of people who must have eaten con- taminated meat. The panic caused by mad cow disease has raised awareness of all the prion diseases. Perhaps, the emergence of new dis- eases, such as BSE and vCJD, might be related to the ways in which human beings have affected the environment, especially through global exchanges of previously isolated plants, animals, and infectious agents.
In 1972, after one of his patients died of CJD, Stanley Prusiner
began studying the literature that linked CJD to kuru and scrapie. Creutzfeldt–Jakob Disease appears to strike sporadically, affecting about one in one million people over the age of 60 years throughout the world. When Prusiner isolated the scrapie agent from the brains of diseased hamsters, he was surprised to ﬁnd that it apparently con- sisted of a speciﬁc protein. All previously known infectious agents, even the smallest viruses, contained genetic material in the form of nucleic acids, either DNA or RNA. Prusiner’s ‘‘protein only hypothesis’’ was initially regarded as heresy, but within a few years, genes that encoded prion proteins were found in all animals tested, including humans.
Despite continuing skepticism and controversy, by the early 1990s
many scientists had accepted Prusiner’s prion hypothesis. In 1997, Prusiner was awarded the Nobel Prize in Physiology or Medicine for
discovering prions and establishing a new genre of disease-causing agents. According to Prusiner’s theory, prion proteins can exist in two distinct conformations, one of which is quite harmless. Prion proteins can also exist in altered conformations that act as rogue proteins or
‘‘evil twins.’’ In the altered conformation, prion proteins are apparently capable of inducing their benign counterparts to undergo the same transformation. As the transformed proteins accumulate and aggregate, they form thread-like structures that ultimately destroy nerve cells and result in fatal brain diseases. Despite many uncertainties about the way in which prions cause brain disease, Prusiner suggests that understanding the three-dimensional structure of prion proteins might lead to useful therapeutic interventions. Moreover, the success of the prion hypothesis in explaining infectious, heredity, and sporadic forms of scrapie-like diseases suggests that similar mechanisms might play a role in other disorders, including Alzheimer’s disease, Parkinson’s disease, and amyo- trophic lateral sclerosis.
In 1972, Sir Frank Macfarlane Burnet, the Australian virologist
who shared the Nobel Prize with Peter Medawar in 1960, famously declared that ‘‘the most likely forecast about the future of infectious dis- ease is that it will be very dull.’’ Since the 1960s, many physicians and health policy analysts shared the assumption that microbial diseases could be essentially ignored because of the power of antibiotics, vaccines, and other therapeutic agents. By the end of the century, it was clear that predictions about the demise of infectious disease had been grossly exaggerated. Infectious organisms—known and previously unknown—continued to evolve and ﬁnd ways to exploit new oppor- tunities. By the end of the twentieth century, approximately ﬁve hundred million illnesses and six million deaths each year were caused by AIDS, tuberculosis, and malaria. One out of every two deaths in developing countries is due to infectious diseases, but globalization and rapid transportation link all parts of the world.
The ‘‘catalog’’ of human diseases is likely to grow as new diseases
appear and old categories, such as ‘‘fevers’’ or ‘‘fevers of unknown ori- gin,’’ are re-examined and broken down into speciﬁc ‘‘new’’ diseases. The appearance of West Nile virus in New York City in 1999 and its subsequent spread into other states demonstrated how easily pathogens could establish themselves in new regions. Previously unknown diseases, such as AIDS, Legionnaires’ disease, Lyme disease, mad cow disease, Ebola fever, Rift Valley fever, SARS, avian inﬂuenza, monkey pox, Nipah virus, Lyssavirus, Chandipura virus, and so forth, have appeared and old diseases have spread to new areas while many pathogens have become antibiotic-resistant. For example, antibiotic-resistant strains of Staphylococcus aureus have caused fatal pneumonias, heart infections, toxic shock syndrome, and necrotizing fasciitis (ﬂesh-eating bacteria).
Scientists have identiﬁed many factors that may affect the distri- bution and emergence of infectious diseases, including environmental factors, population growth and age distribution, migration, war, inter- national travel and commerce, technological and industrial factors, and national and international commitment to disease control and pub- lic health measures. Climate changes produced by global warming might have signiﬁcant consequences for the global distribution of diseases, especially water-borne diseases and diseases transmitted by mosquitoes. Although the chronic, degenerative diseases of old age have become the major concern of the wealthy, industrialized countries, in much of the world, poverty and the lack of basic sanitary facilities contribute to the continuing burden of infectious diseases. According to the United Nations, at the beginning of the twenty-ﬁrst century more than one billion people lack clean drinking water and more than two million people die each year from illnesses associated with dirty water and poor sanitation.