Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 31

Reconstruction of the prehistoric past to provide a plausible account of when or how viruses established themselves in human populations is challenging. However, extrapolating from current knowledge, we can deduce that some modern viruses were undoubtedly associated with the earliest precursors of mammals and coevolved with humans. Other viruses entered human populations only recently. The last 10,000 years of history was a time of radical change for humans and our viruses: animals were domesticated, the human population increased dramatically, large population centers appeared, and commerce and technology drove worldwide travel and interactions among unprecedented numbers of people.

Viruses that established themselves in human populations were undoubtedly transmitted from animals, much as still happens today. Early human groups that domesticated and lived with their animals were almost certainly exposed to different viruses than were nomadic hunter/gatherer societies. Similarly, as many different viruses are endemic in the tropics, human societies in that environment must have been exposed to a greater variety of viruses than societies established in temperate climates. When nomadic groups met others with domesticated animals, human-to-human contact could have provided new avenues for virus spread. Even so, it seems unlikely that viruses such as those that cause measles or smallpox could have entered a permanent relationship with small groups of early humans. Such highly virulent viruses, as we now know them to be, either kill their hosts or induce lifelong immunity. Consequently, they can survive only when large, interacting host populations offer a sufficient number of naive and permissive hosts for their continued propagation. Such viruses could not have been established in human populations until large, settled communities appeared. Less virulent viruses that enter into a long-term relationship with their hosts were therefore more likely to be the first to become adapted to reproduction in the earliest human populations. These viruses include the modern retroviruses, herpesviruses, and papillomaviruses.

Evidence for knowledge of several diseases that we now know to be caused by viruses can be found in ancient records. The Greek poet Homer characterizes Hector as “rabid” in The Iliad (Fig. 1.3A), and Mesopotamian laws that outline the responsibilities of the owners of rabid dogs date from before 1000 B.C.E. Their existence indicates that the communicable nature of this viral disease was already well-known by that time. Egyptian hieroglyphs illustrate what appear to be the consequences of poliovirus infection (a withered leg typical of poliomyelitis [Fig. 1.3B]). Pustular lesions characteristic of smallpox have also been found on Egyptian mummies. The smallpox virus was probably endemic in the Ganges River basin by the fifth century B.C.E. and subsequently spread to other parts of Asia and Europe. This viral pathogen has played an important part in human history. Its introduction into the previously unexposed native populations of Central and South America by colonists in the 16th century led to lethal epidemics, which are considered an important factor in the conquests achieved by a small number of European soldiers. Other viral diseases known in ancient times include mumps and, perhaps, influenza. Europeans have described yellow fever since they discovered Africa, and it has been suggested that this scourge of the tropical trade was the basis for legends about ghost ships, such as the Flying Dutchman, in which an entire ship’s crew perished mysteriously.

Humans have not only been subject to viral disease throughout much of their history but have also manipulated these agents, albeit unknowingly, for much longer than might be imagined. One classic example is the cultivation of marvelously patterned tulips, which were of enormous value in 17th-century Holland. Such efforts included deliberate spread of a virus (tulip breaking virus or tulip mosaic virus) that we now know causes the striping of tulip petals so highly prized at that time (Fig. 1.4). Attempts to control viral disease have an even more venerable history.

Figure 1.3 References to viral diseases from the ancient literature. (A) An image of Hector from an ancient Greek vase. Courtesy of the Penn Museum, object 30-44-4. (B) An Egyptian stele, or stone tablet, from the 18th dynasty (1580–1350 B.C.E.) depicting a man with a withered leg and the “drop foot” syndrome characteristic of poliomyelitis. Image courtesy of SPL/Science Source.