The Adaptive Parasite
 
Meghan Rothenberger


Viruses are molecular sharks, a motive without a mind.  They have sorted themselves into tribes, and they infect everything that lives. . . . Unknown viruses are coming out of the equatorial wildernesses of the earth and discovering the human race. . . . You might call AIDS the revenge of the rainforest. (Preston 160-61)

After reading Richard Preston's ominous and threatening portrayal of viruses in his article "Crisis In The Hot Zone," one may be alarmed enough to invest in surgical scrubs and a space suit to wear as a permanent precaution against these evidently vengeful creatures.  In truth, there are lethal viruses that exist for which there is currently no vaccine or cure, and there are various emerging viruses that are infectious to humans. However, despite this unfortunate and frightening reality, Richard Preston and other creative writers may be presenting a misleading depiction of a virus' actual niche within the global ecosystem, modes of infection, and relationship with humans.  Science fiction novels and movies like "Outbreak" encourage the public to view viruses as microscopic monsters that exist to ultimately put an end to the human population, but viruses are not motivated by either logic or instinct to kill.  In fact, viruses are not motivated at all.  They have merely evolved to survive by utilizing the raw materials that the ecosystem provides.  Viral pathogens have developed, through the selective process of evolution, to exist as parasites, and the inevitable ecological interaction between organisms, the ignorance and negligence of human behavior, and the cultural habits and customs of humanity has enabled a multitude of viruses to emerge and thrive within our population.

Viruses are simple, microscopic organisms; however, they are no exception to the evolutionary function of natural selection. The process of evolution results when organisms reproduce more individuals than the environment can support, genetic variation naturally and inevitably occurs, the organisms that are the most successful within their habitat survive, and those that are deficient and weak are eliminated. The survivors of this ecological pressure are able to flourish, reproduce, and pass their favorable genes onto subsequent generations. Many biologists believe that viruses evolved after the appearance of the first cells and favor the hypothesis that they "originated from fragments of cellular nucleic acids, [DNA or RNA], that could move from one cell to another" within an organism (Campbell, Mitchell, and Reece 330).  This theory asserts that a portion of DNA or RNA initially became separated from the remainder of the cell's genome, and the newly formed virus then migrated into another cell. Isolated fragments of DNA or RNA would be biologically inert; therefore, viruses have been "engineered by the forces of evolution" to maximize their survival as mobile, parasitic organisms  (Preston 159).  Viruses, as mere segments of nucleic acids, have developed various adaptations that ensure their reproduction, their transport from cell to cell, and their survival at the expense of another organism.

Viruses have adapted a dependency on the cells of another organism, termed a host, to provide both shelter and a metabolic means of reproduction.  Different viruses have evolved various mechanisms to survive, and there are exceptions to the typical viral life cycle. However, in general, they are obligate intercellular parasites that insert their genetic material into a host cell, force the host cell to cease its own metabolic processes, and regenerate by utilizing the cell's components.  After the viral parasite has diverted the host's resources for the reproduction and assemblage of new viruses, the viral invasion often leads to the death of the host cell as a result of the emergence of the virus' offspring.  When the newly constructed viruses emerge from the infected host cell, they must gain the potential to move to and to infect additional cells to maximize their ability to produce mass quantities of viral offspring.  Otherwise, the viral life cycle would remain in a state of indefinite standstill because an isolated virus is unable to replicate itself.

Although they are the "simplest of all genetic systems" and are composed of "little more than aggregates of nucleic acids and proteins," viruses have, through the selective process of evolution, developed an efficient means of mobility  (Campbell, Mitchell, and Reece 324).  In fact, "[the] evolution of genes coding for capsid proteins, that protect the viral genome and aid in its transmission, may have facilitated the infection of [alternative] cells" (Campbell, Mitchell, and Reece 330). Although some viruses are more complex than others are, all viruses are composed of some form of nucleic acid, DNA or RNA, and a capsid.  A capsid is the protein shell that encloses the viral genome, and capsids are responsible for a number of functions including protecting the viral genome, disguising the viral genome and preventing it from being detected and destroyed by the host's immune system, and enabling the virus to bind to and enter a host cell.  In addition to capsids, some viruses, like HIV, are equipped with viral envelopes, outer membranes that exist externally to capsids, to further aid the pathogen in transmission.  Viral capsids and envelopes facilitate transmission not only from one cell to another within an organism but also from cells within one organism to cells within another organism.

The success of a virus depends on the ability of the virus not only to produce large quantities of mobile offspring but also to be versatile, and evolution has, as a result, also selected viral genomes capable of versatility.  If a virus is capable of adapting to survival within multiple species of host organisms, then it gains an automatic advantage.  For instance, "[swine] flu virus . . . can infect both hogs and humans, and the rabies virus can infect a number of mammalian species, including rodents, dogs, and humans"  (Campbell, Mitchell, and Reece 327). Consequently, viral reproduction is no longer limited, and the virus is provided with an innumerable amount of opportunities to thrive within its environment.

Because of the evolution of versatile viruses, human behavior often provides viruses with targets of opportunity.  The transmission and emergence of human viruses are often caused through "ecological disruption resulting from human activities"  (Le Guenno 58).  Ecosystems are complex entities and have many interactive components.  They are composed of all the living organisms and nonliving factors of a given area and their interaction and effect on one another.  All organisms maximize their ability to survive by evolving to fill a distinct niche within their ecosystem.   All species of viruses, like other organisms, have evolved to successfully maintain themselves within an ecosystem of interactive organisms.  When human action causes environmental change, humans become responsible for creating contacts with viral pathogens and providing them with an opportunity to acquire residence and propagate themselves within yet another host.  The versatility and adaptability of viruses allows their transmission into humans, and humans facilitate this transmission by engaging in behaviors that put themselves in a position to be infected by viral parasites.  A multitude of hosts to infect ensures a virus' survival better than does infecting only one host.  Viruses are not particular, and humans provide as good a host as any.

Many individuals view viruses as terrorizing creatures that intend to wipe out the human species, and they are unaware that their negligent behaviors are "[facilitating] contacts with animals carrying viruses pathogenic to humans" and allowing, possibly even forcing, viruses to infect humans (Le Guenno 58).  Deforestation, agricultural cultivation of large areas of land, and laboratory research that requires the transfer of various wild animals from their natural habitat to a laboratory are responsible for causing many outbreaks of viral infections.

Machupo, Junin, Sabia, Guanarito and other arenaviruses that cause hemorrhagic fever all emerged as a result of a human action that caused an ecological change.  Arenaviruses "circulate naturally in various populations of animals, . . . [and epidemics] are . . . linked to the presence of animals that serve as reservoirs for the virus and sometimes as vectors that help transfer it to people"  (Le Guenno 57).  Although arenaviruses are known as "emerging viruses," these viruses are not at all new.  In fact, they have existed for millions of years within populations of animals, and these viruses emerge only when environmental conditions are altered or disrupted, usually as a result of a human action.  For instance, in Argentina, humans facilitated contacts with vesper mice, the animals in which both the Machupo and Junin viruses reside, when farmers began to cultivate areas of maize.  Vesper mice thrive within this habitat, and, when human machinery "[suspended] clouds of infective dust . . . [and created] an aerosol of infective blood when they accidentally [crushed] the animals," the agricultural workers became satisfactory opportunistic hosts for these deadly arenaviruses (Le Guenno 58).  These agricultural workers became infected with the illness hemorrhagic fever, which causes bleeding, cardiovascular, digestive, and neurological complications, and, eventually, massive hemorrhages, organ failure, and death.  Furthermore, a similar viral outbreak resulted in a community that began to clear a forest. During the deforestation process, humans became infected with the Guanarito virus when they came in contact with infectious dried urine and excrement of the virus' primary host, the cotton rat.  Once again, viral emergence and transmission to humans occurred when a human action disrupted the environment and facilitated contacts with the cotton rat, an established resident of the forest and a carrier of the Guanarito virus.

The Rift Valley fever virus is another example of a pathogen that emerged as a result of a human action that caused an ecological disruption.  Rift Valley fever infected an extremely large number of humans after the construction of the Aswan Dam.  The construction of this dam caused water levels to rise and created a prime environment for the propagation of large numbers of mosquitoes. Mosquitoes are the vectors that carry and transmit the Rift Valley fever pathogen. The deadly combination of human settlement in that area and swarms of mosquitoes facilitated contacts between these organisms and permitted the spread and outbreak of this virus.

Finally, Ebola and other viruses are transferred to humans during laboratory research on infected chimpanzees and other monkeys. Many biological industries attempt to produce vaccines from animal cells; however, in some cases, the animal cells are contaminated by disease causing pathogens.  It is possible that these pathogens could be transmitted to those vaccinated or to those preparing the vaccine.  For example, in Marburg, Germany, "[twenty-five] people fell ill after preparing cell cultures from the blood of vervet monkeys"  (Le Guenno 60). This incident, in 1967, resulted in not only the death of seven individuals but also the death of all of the laboratory monkeys, and the death of the monkeys suggests that they are not the natural vectors of the Marburg pathogen.  Unfortunately, "neither the reservoir nor the natural modes of transmission have been discovered [for this pathogen]"  (Le Guenno 60). This mode of infection was also illustrated on the Ivory Coast when a researcher became infected with Ebola while handling blood samples from chimpanzees.

Although all of these viruses may have varying degrees of severity and different vectors or primary hosts, they share one commonality.  These pathogens did not jump out of their hosts into humans or escape from their natural habitat in search of a human population to conquer but were spread as a result of a human behavior that caused ecological change to their environment.  In each incident of a viral outbreak, humans neglected to consider the possible dangers that viruses may present and, in anticipation of this possibility, to establish some means of preventing viruses from infecting the human population.

Human behavior and interaction is also largely responsible for influencing the evolution of a virus' severity and virulence. Evolutionary biologists recognize that viruses have evolved two major modes of transmission that depend on the parasite's relationship with its host. Some species of viral pathogens have evolved to only mildly impair the host organism.  If human behavior inhibited a virus' opportunity to spread and created slow transmission rates for viral pathogens, then the virus would have to depend solely on the target cells of its human host until it was provided with a means of infecting another individual.  The less virulent virus would benefit by adapting a life cycle that did not rapidly destroy all of the cells it depends on for reproduction.  A virus of this type would also gain an advantage through latency until given the opportunity to infect other individuals.

Sexually transmitted infections and vertical infections passed from mother to child are examples of viral species that have evolved to become less virulent.  These pathogens must maintain themselves within their host by remaining dormant and allowing the host to remain healthy long enough to eventually participate in activities that will ensure its transmission.  Sexually transmitted viruses depend on the promiscuity of the host, and vertically transmitted viruses depend on a healthy and fit mother to attract a mate, become pregnant, and give birth to an equally healthy, infected infant. AIDS and Hepatitis B are viruses that are transmitted both sexually and vertically.  Although AIDS is extremely lethal and kills virtually all of the humans it infects, the virus that causes AIDS, HIV, is less virulent because it maintains itself within the host, remains dormant, and does not immediately engage in rapid replication in order to allow a sufficient amount of time to be passed to another human host.  Because the health of the infected individual does not immediately deteriorate, he or she is capable of engaging in activities that would allow the transmission of the virus to another human.  In this way, HIV has evolved to become frighteningly successful as a less virulent virus.  In the case of Hepatitis B, the virus typically remains dormant for an even longer period of time than AIDS.  The Hepatitis B virus maintains itself within its host long enough for an infant, infected by its mother, to develop and reproduce infected offspring of its own.

Conversely, the second category of viruses has evolved to become extremely virulent.  Virulent pathogens do not depend on the mobility of their host to transmit the infection, and they generally rely on a primary vector, other than a human, for transmission. These viruses reproduce rapidly causing intense concentrations of themselves within the host.  If the pathogen were provided numerous opportunities to pass quickly to other individuals, then this type of pathogen would achieve success by virulence, which allows it to take maximum advantage of each of these opportunities. These infections are often more deadly, for they do not require the host's health or appearance of health for transmission.  Because interaction has provided these viruses with a multitude of cellular factories in which they can propagate themselves, the primary host becomes insignificant and thus disposable.  For example, viruses traveling by means of other vectors, such as insects or sewage, are often highly virulent and lethal. The epidemic of Yellow Fever, caused by the Amaril virus, that broke out in Philadelphia in 1793 was extremely virulent and lethal to humans because mosquitoes were responsible for transmitting the genes of this infectious agent.  This epidemic resulted in the death of four thousand people and drove Washington, Jefferson, and other government officials from Philadelphia, the capitol in 1793.   In contrast to the extended dormancy of HIV, the incubation period of the Yellow Fever virus is merely three to six days.  The onset of symptoms, like fever, vomiting, hemorrhaging, delirium, and coma, is relatively sudden.

Furthermore, the modern cultural habits and ignorance of the human civilization continues to elevate the virulence of certain viruses and may have, either directly or indirectly, caused viral diseases like AIDS to blossom into a worldwide pandemic. For instance, the documented results of a collaborative study on HIV virulence revealed that

frequent HIV transmission events . . . and rapid human-to-human passage linked to a marked increase in newly infected people.  HIV virulence also increased because the transmissions occurred when donors had high virus loads, so recipients received large populations. . . . [However, a] turning point . . . reduced the number of sex partners and . . . resulted in fewer new infections and declining virulence . . . and produced a somewhat less aggressive epidemic.   (Goudsmit 9)
This investigation demonstrates that frequent opportunities to pass sexually allow the HIV retrovirus to become extremely virulent.  When humans are promiscuous, the virus reproduces rapidly to maximize its transmission to other humans.  Because human interaction provides this virus with many hosts in which it can reproduce and survive, each infected individual becomes increasingly unimportant; thus HIV becomes more deadly.  Conversely, when opportunities for transmission are decreased, this trend is reversed, and HIV becomes less virulent.  Much of humanity remains unaware of the HIV virus' actual modes of transmission and infection and, as a result of human ignorance, many individuals continue to engage in risky behaviors that allow the virus to thrive and multiply within the human population.

In view of a virus' actual evolution to exist as a parasite, incidents of emergence, and modes of transmission, one can disregard his or her prior notion of viruses as murderous, conniving, and bloodthirsty monstrosities.  A thorough evaluation of viral pathogens immediately unveils the reality of viral existence within our population.  It is the interaction between organisms and the negligent behavior and common habits of humanity that ultimately allows viruses to emerge and infect the human species.  Viruses are not at all motivated by an intense desire to eliminate humans as Richard Preston suggests in "Crisis in The Hot Zone." In most cases, rather, it is the ecological disruption caused by humans, like deforestation, agricultural cultivation, and laboratory research, that facilitates the transmission of viral infections to humans.  Furthermore, humans may be responsible for increasing the severity and virulence of some viruses, like HIV, as a result of negligent behavior that provides the pathogen with frequent opportunities to be transmitted.  Although an attempt to reverse the progression of human behavior is unrealistic, it is still possible to educate humanity about the modes of infection of viruses and to create a global awareness about the dangers of viruses so that people will begin to take some precautions against these deadly, microscopic organisms.  For instance, in the case of AIDS, we cannot expect to eliminate promiscuity or drug use; however, we can encourage protected sex and the use of clean needles.  In addition, it would not be wise to eliminate the scientific advances provided by laboratory research on various animals or the benefits of constructing a dam or canal. However, we could, for example, take measures to control the massive propagation of mosquitoes in the event of the construction of a dam or canal.  This relatively simple precaution will reduce the occurrence of diseases, such as Rift Valley fever and Yellow Fever, and may save thousands of lives.  Although it is virtually impossible to completely prevent viruses from infecting humans, humanity's best defense against viral invasion is communication, education, and awareness to promote cautious behavior and to reduce the number of further outbreaks of deadly viruses.