April 24, 2014

WHAT’S UP DOC?: What Is a Virus (And Why You Should Care)?

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Right now, there is a measles outbreak in New York City. There’s also one in Orange County, California, which is the “worst officials have seen in two decades.” There have been measles scares in Boston and the Bay Area too. There are all sorts of bizarre outbreaks of measles, mumps and more in the United States and around the globe, as depicted in a fantastic (and terrifying) infographic from the Council on Foreign Relations. I urge you to check it out and take note of the title above the map: “Vaccine-Preventable Outbreaks.” There is no question that rare infections are on the rise again due to the ill-advised anti-vaccine movement, which is based on a since-retracted study from 1998 linking the measles/mumps/rubella vaccine to autism.

If any of your Facebook friends are interested in immunology (or interested in not getting measles), odds are you’ve seen all sorts of articles recently about these anti-vaccine-related viral outbreaks (perhaps even the one from the Onion). If you’ve read them, you may have noticed that these articles typically explain where the outbreak is, what the symptoms look like and the importance of getting the vaccine. This is, of course, great. But all of the articles I’ve encountered have forgotten something pretty important.

Actually explaining what a virus is.

I concede that teaching the general public Virology 101 is not the purpose of most news articles, and that the more important objective is getting parents to vaccinate their kids again. It is also true that explaining what a virus is may be easier said than done. But the puzzling and powerful nature of viruses is exactly why we should all want to know more about them.

A virus consists of a piece of genetic material (DNA or RNA) packaged inside of a protein coat, with some viruses having a lipid envelope surrounding this coat. That’s it. At the end of the day, the infectious particles that kill human beings around the globe are teeny-tiny hunks of protein with some nucleic acid inside. To appreciate the scale we’re talking about, the measles virus is just 220 nanometers, which is approximately 409 times smaller than the average thickness of a piece of paper. Other viruses are even smaller; there are sites that let you compare a few different viruses with objects from daily life, bacteria and various cellular components, which really drives home just how small these bad boys are.

With such diminutive size and so few components in their possession, viruses can’t really do much without the help of a host cell. A host could be you, me, farm animals or a plant, depending on what virus we’re considering (some of the scariest viruses are ones that can jump from animal to human hosts, or zoonoses, but this is a topic for another time). Once a virus has found a suitable host, then it can fulfill the goal of all species: replicate. And here’s where scientists of the past have often been stymied by viruses. Since a virus can’t reproduce on its own, does it count as “alive?” How do we define “alive” anyway?

For an in-depth discussion of this very question and on the history of how viruses are classified, Scientific American gives a good overview. Bottom line, viruses are widely considered nonliving since they cannot independently utilize energy, make proteins or reproduce, but their ability to co-opt the machinery of a bona fide living cell for these purposes places viruses at the border of living and nonliving. But just how can a tiny molecular suitcase filled with such simple baggage fool our sophisticated immune systems?  Why haven’t our defense mechanisms eliminated these nonliving entities?

Understanding exactly why viruses exist is likely to always be a bit of a mystery. My favorite theory is the virus-first hypothesis, which suggests that viruses arose before cellular organisms and then co-evolved with them (more on this and other hypotheses can be found here). To me, this is the best argument due to the evolutionary benefit of a method of horizontal gene transfer, or the sharing of genetic material between organisms without the lengthy process of mating, creating offspring and raising those offspring. Perhaps at one time, having viruses helped quickly create genetic diversity within our species, and thus viruses stuck around and co-evolved with us. Supporting evidence can be found by inspecting our genomes, where you can find that eight percent of the human genome derives from retroviruses which inserted themselves into our DNA and then got passed down by our ancestors. Humans and other mammals have in turn utilized those ancient viral genes to assist in development, formation of the placenta and who knows what else.

So perhaps viruses have not been eliminated because sometimes they have proven helpful to our species. But what about all those times when they really aren’t helpful, like the outbreaks we’re seeing now? You would think that our complex immune system would be able to handle any virus that comes along. But, as it turns out, viruses have evolved brilliant mechanisms to evade the immune system. There are viruses with proteins on their coats that mimic human proteins, so our system doesn’t recognize them as foreign. Other viruses prevent infected cells from presenting pieces of degraded viral proteins on their surface, effectively stifling that cell’s cry for help to the immune system. Others, like HIV, preferentially infect cells of the immune system, so that when new cells traffic to an infected cell to come take care of the problem, they become infected themselves. You could write an entire book about the many impressive strategies that viruses use to survive the tumultuous world inside of our bodies. And they accomplish this all with just a few genes. Suddenly, these tiny invaders don’t seem so simple anymore.

Hopefully, examining what a virus is, why we have them and how impressive they are will prove helpful in the effort to increase understanding of how crucial it is that all those who can receive vaccinations do so. Viral infection should not be treated lightly. Perhaps as a remnant of an ancient symbiotic relationship with viruses, they are very much here to stay. There’s a lot we don’t currently understand about viruses, but we do know that these “creatures” are capable of elegant ways to avoid elimination by our immune system.  If we want to keep viruses at bay, we need to strive to teach more people about them with the hope that doing so creates renewed respect for medical advances that turn the tables and allow us to evade them.

Meredith Wright is a first-year Ph.D. student at Weill Cornell Medical College.  She can be reached at mew2019@med.cornell.edu.  What’s Up Doc?  appears alternating Fridays this semester.

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