when most people hear the word “virus”, the first image that comes to mind is generally something along the lines of a sick person, an epidemic, trips to the doctors office, vaccinations, or, for those with some biology background, a crystalline, nightmarish spider-alien injecting DNA into a defenseless cell. viruses are generally perceived as perpetrators of malaise, a scourge to society that modern science can and will eventually eradicate. only in the past decade, since the advent of fast and relatively cheap genetic sequencing technology, have scientists begun to recognize the staggering diversity of viruses in the world, many of which are entirely benign and have no known ecological function. the dawning realization that really are just about everywhere- they are ten times more abundant than bacteria in the ocean- indicates an incredibly effective strategy for self-propagation. this strategy in turn represents a form of existence so simple that scientists have been debating for decades whether or not viruses can be classified as life.
despite their apparent simplicity, understanding viruses has been one of biology’s greatest challenges since the beginnings of the molecular revolution. the traits that we have discovered to be ubiquitous among viruses are relatively straightforward. generally, a virus consists of a single piece of naked DNA, encapsulated in some sort of protein-based coat. viruses cannot be considered cells because they contain none of the internal machinery necessary for growth or self-replication. instead, many viruses replicate by inserting their DNA into the cells of a host. this invading DNA is able to co-opt the host cell’s own replication proteins, and turn the host into a small factory for new viruses.
many but not all of the viruses that cause human disease use this strategy, and they often do so with alarming efficiency. another common viral replication strategy is to insert DNA into a host, and integrate that DNA into the hosts own DNA. viruses that employ this strategy are effectively choosing symbiosis inside a host, and replicate themselves in step with the host cell’s own cycle.
it may seem strange that some viruses act aggressively- invading, replicating and moving on once they have plundered all the resources available, while others choose a life of harmless symbiosis within their host. how can we come up with a general definition for all viruses if this is the case? shouldn’t we classify these critters as two unique types – neither truly alive perhaps, but fundamentally different in their non-living existence?
to answer this question, one must think carefully not about what viruses are doing but why. in both cases, a fragment of DNA is simply trying to replicate itself in the most effective way possible. for some, this means integrating itself into an organism, and reproducing in concert with the organisms own generations. for others, it means rape, kill, pillage and burn. viruses are the ultimate narcissists- no ambitions for complex structure or function,, simply a raw, unabashed need for self-propagation.
if existence driven entirely by the need to replicate and produce more of oneself what it means to be a virus, i don’t think it’s a far stretch of the imagination to draw parallels with organisms that we officially classify as “alive”. with simple single-celled life, the similarity is easy to see. unicellular bacteria are essentially DNA vessels, but with extra compartments for the tools and machinery required to replicate. some single-celled bacteria do reproduce sexually and, in exchanging DNA, produce offspring that are not genetic clones. nevertheless, the idea is essentially still simple propagation of genes, but given one more level of complexity in that the replication process is self-sufficient.
but how much similarity can there possibly be between a complex, multi-cellular organism, and a single replicating strand of DNA? try thinking of a complex organism, like a cat, horse, or even human, as a nation of cells. each cell is an individual citizen, and each citizen has a specialized job that he must perform as an effective member of the community. if too many citizens dissent, or get lazy, and choose not to perform their allocated jobs, the community falls apart. and what do these citizens, many of whom look and act very different, and would certainly never be caught getting coffee or drinks together, all have in common? dependency on each other for replication.
a human being is orders of magnitude more complex than a virus, and I am not trying to diminish that complexity, or even to claim that it can be reduced to aggregate of cells driven by a simple process. but the common purpose of genes, in everything from their rawest form that do not even consider living, to the most complex organism evolution has produced, speaks to the ancestry we all share.