H1N1 (inappropriately called swine flu) is a strain of influenza that was produced by its environment -- selection for increased virulence in infected individuals and for low virulence to facilitate spread. The influenza virus has an RNA genome. That means that it has a rather rudimentary enzyme for making copies of its RNA to make new viruses. The simplicity of the enzyme means that it makes lots of mistakes. In fact the error rate of virus RNA polymerases is almost a million times higher than the DNA polymerase complex that replicates human chromosomes with great fidelity. Essentially every virus produced is a mutant (new RNA sequence). Which of these mutants prevails is a classic example of evolutionary natural selection.
Viral Evolution is Fast
The rate and direction of evolution is dependent on the generation time and selection. Viruses replicate on the scale of hours, and evolution is fast -- during the time it takes to incubate and pass on to the next host. Selection, differential increase of one viral combination of genes vs. others, is provided by conditions within the host (temperature, antiviral drugs, inflammation) and between the current host and the next victim (rapid transit, mobility of infected, contact, aerosol transmission, vectors, isolation).
H1N1 Evolution Is Dependent on Choices
The course of a pandemic is determined by public health choices that control the evolution of the viral pathogen. With the current H1N1 pandemic, the use of antiviral drugs, such as Tamiflu, and the control of contact between infected and potential new hosts, will determine if hundreds or millions die. Effective isolation of infected individuals and use of antivirals and symptom relief without transmission of treated virus to others, could eliminate H1N1 as a threat.
Selection within Hosts Increases Virulence
The most virulent, the viruses that reproduce and spread to new cells most aggressively, always win. This is simple natural selection. The total resources of a body are limited and the mutant virus that utilizes the resources the fastest, makes the most copies of itself. Selection between genetic variants or mutants within the same organism leads to increased virulence.
Antiviral Drugs Will Be Useful Until Improperly Controlled
Antiviral drugs, such as Tamiflu that attacks H1N1 neuraminidase, provide high within-host selection pressure that favors mutants that are resistant to Tamiflu. If viruses from Tamiflu-treated patients are permitted to spread to others, the usefulness of Tamiflu will be very brief and more virulent, Tamiflu-resistant strains will quickly predominate.
Selection between Hosts Decreases Virulence
Rapid killing of the host prevents spread of a virus through a susceptible population. Virus strains that do not interfere with mobility have a selective advantage, because they readily spread to more individuals. Thus, the majority of influenza strains that spread throughout the world each year have mild symptoms, because measures to block dissemination are not taken. Populations are exposed to many different viral antigens and adapt immunologically as the virus evolves.
Global Transportation Leads to Extreme Virulence
Ease of transport reduces the selective disadvantage associated with physical incapacitation and selects for increasing virulence. Rapid, long distance transportation between large, dense populations provides a selective advantage to highly virulent, deadly strains. Even with a brief incubation and rapid, lethal symptoms, the most virulent strains will have no counter-selection toward lower virulence imposed by the need for physical mobility to spread to new hosts. Under these extreme conditions, healthcare practitioners will predominate as victims, as the pandemic spreads from hospital to hospital in different regions of the world until it is blocked by contamination prevention.
Isolation and Blocking of Severe Symptoms
If each infected individual is prevented from spreading the evolving virus to new hosts, then the rapid evolution of viruses is not a problem. Within a week, the body produces antibodies and other immunological adaptations that eliminate the virus. If extreme host response, such as a cytokine storm, is blocked, and secondary infections are prevented, then the virus is of minimal concern.
Isolation of infected individuals is of primary importance and this is the goal of medical advice to stay home and out of contact with others if you have any flu symptoms, and wash your hands frequently.
reference:
Why We Get Sick: The New Science of Darwinian Medicine by Randolph M. Nesse and George C. Williams
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