Ebola, Dengue (hemorrhagic and not), Measles, HIV, Smallpox, Polio, Influenza, Coronaviruses (SARS/MERS), Caliseviruses (Rotavirus), Hanta virus, Rabies, these are all killers. Viruses are, perhaps, the least well understood pathogens by the general public, and handily the most feared. You only need to turn on the TV to see various plots starring novel superviruses that threaten to obliterate the population, dastardly villain optional, to take my point. They're fundamental horror at its finest, an invisible agent that turns those closest to you into death machines that shed millions of patent virus particles during the course of their infection. Only very recently, with vaccines and improvements in hygiene, nutrition, and medical care, have we begun to curb the viral threat to the population.
So, what is a virus? What makes them distinct from those other, dastardly, deadly microbes, bacteria? First off, they're not generally considered to be living organisms. That's right, because living organisms have metabolic processes, they take in nutrients and put out waste, they synthesize proteins, replicate genomes, and respond to their environment. Viruses don't do any of these things- at least, not on their own. Viruses don't have a proper nucleus, nor do they come with organelles, or much more than their genetic data and what's required to get it in a cell and replicating. Outside the cell, a virus is not much more than a static particle, floating about in Brownian motion, waiting to encounter its target cell. Once inside the cell, however, the viral proteins, enzymes, and genetic data get to work hijacking the cell's transcription (sometimes, with DNA and Retroviruses) and protein synthesis machinery to do one thing: make more virus.
The scary bit is that there's evidence to suggest that the diversity of species offers a barrier against pathogen spread into human populations by offering ectoparasites (mosquitoes, ticks, fleas, etc) and other vectors choices besides people. [3] With biodiversity falling, and human populations rising, it may only be a short time before we experience our next major pandemic. There are antiviral therapies available, but acute infections are often too far progressed by the time severe symptoms manifest for the antivirals to make a difference, and some viruses can easily select for escape mutants that aren't at all effected by the pharmaceutical therapy.
How do viruses make you sick? The mechanisms vary, but for the most part, it's not actually the virus making you feel sick, it's you. That's right, your symptoms- aches, chills, fever, feeling crappy- are all the result of your immune system trying to reign in the viral infection. The only problem is that it sometimes goes a bit overboard, and ends up carpet bombing the city to catch the thief, so to speak. For example, there's a hypothesis circulating that type 1 Diabetes Mellitus is actually the product of an autoimmune response secondary to a viral infection. The hypothesis implies that the infection itself is either fairly minor (such as a cold) or completely inapparent. The immune system successfully controls and clears the virus, but, possibly due to a similarity between viral antigens and surface proteins on the insulin secreting cells of the pancreas, the immune system goes on to attack and destroy the body's insulin factory. Take it with a grain of salt, since there's been no peer reviewed research that I'm aware of that positively identifies a viral culprit, but the hypothesis is sound, in my opinion.
All viruses are bad, right? Not quite. There's evidence to suggest that roughly 8% of our genome is viral in origin.[1] That is to say that, at some point, a virus managed to successfully insert its genome into ours, and get its genetic data incorporated and passed from generation to generation. So what, you may ask. Placenta, I will answer. There's evidence that suggests that mammals gained placental function secondary to a retroviral integration event. That's right, you may have a virus to thank for not laying eggs. While it is a very good thing that we've managed to save millions of lives through successful vaccination programs, the eradication of smallpox, and the near-eradication of Polio, one can't help but wonder what they might have become in time. Given another ten thousand years, could we have gained a gene from the virus, or through the natural selection caused by the virus? It's hard to speak to anything on it past the level of conjecture, and it's hardly worth the tragic loss of life that would need to occur.
What's more is that we're moving forward with clinical trials in using genetically modified human viruses as cancer killers- and they're pretty darn good at it. The risk of the virus spontaneously mutating into a pandemic monstrosity is generally pretty low. Conjecturally, I'd reckon that it's about the same odds as Rabies becoming a toenail disease and elephants giving birth to sharks. The point is that biological entities don't easily and rapidly become things that they aren't. Nature has a collection of literature on the matter, available for free here: http://www.nature.com/mt/webfocus/oncolytic/index.html
Viruses also show promise as antimicrobials, since they already naturally parasitize bacteria. For more on phage therapy, please visit my article on evolution.
Just remember, just because you haven't found your match yet, doesn't mean you won't meet the virus that's been waiting for you its whole life with your next handshake. Happy valentines day.
[3]http://www.abc.net.au/science/news/stories/s587071.htm
[2]http://www.pnas.org/content/106/29/11827.full
[1]http://www.uta.edu/ucomm/mediarelations/press/2010/01/genome-biologist-reports.php
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