Viruses are not alive. Or maybe they are alive. Or maybe they are somewhere on the edge of life, occupying a space that science has not yet defined.
Viruses defy the usual categories we use to define life. They don’t have cells. They don’t consume, store, or use energy. They don’t move. They don’t reproduce without a host. They don’t show any signs of activity at all unless they’re in an environment where they can spread. They have been compared to nanorobots, and in many ways, it is an apt description. If viruses are not “alive” as we define it, then what should we call these strange evolving and replicating entities?
The “lifecycle” of a virus is bizarre and almost nonsensical, but they play a massive role in human life and death.
What is a virus?
A virus is a type of biologic agent found in almost every ecosystem in the world. They have the barest minimum of structure. The central portion of a virus is their genetic material, which can be DNA or RNA. This genetic material can be wound in loops like bacteria or in strands like humans. Some have single-stranded DNA/RNA and some have doublestranded genetic material, like us.
Their genetic material is encased in a protein shell that is called a viral coat. Many viruses have an outermost lipid layer as well. They can come in all variety of shapes, from the filament-like Ebola to bacteriophages (viruses that attack bacteria) that look more mechanical than biologic. There are countless virus species and just about every type of organism has a corresponding virus that can infect it (even bacteria!).
Since viruses leave no fossils, their origins are still shrouded in mystery. They probably have been around for about as long as cells have. Current thinking is that viruses are either evolved from genetic material that escaped from cells, from cells that reduced their structure to the minimum, from cells that parasitized other cells, or that they have an independent origin and co-evolved with cells.
What viruses do
The dirty secret of all viruses is that they are helpless on their own. Without a host they are as inert as a speck of dust. They are incapable of movement and many virus particles degrade without even getting a chance at replication. Even if something living picks up a virus particle, success isn’t guaranteed. Most viruses have a fairly narrow range of organisms that they can infect. Tobacco mosaic virus only infects tobacco and a few other related plants, so it would be utterly harmless to humans and other animals.
If a virus particle is “lucky” enough to find itself with a suitable host then it’s time to get to work. The first goal of a virus is to get inside a cell. Some viruses get in by trickery, by fooling the pathways of a cell that usually take in nutrients and food. Other viruses fuse their protein coat with the cell membrane, letting their genetic material escape into the interior of the cell. A subset of viruses directly inject their DNA or RNA into an unlucky bacteria cell.
Next, the viral genome hijacks the metabolic pathways of the cell. The infected cell starts producing viral proteins instead of its own. These viral proteins self-assemble into new virus particles. When enough of these new viruses accumulate in a cell, the cell will burst and be destroyed. Some viruses can be released without destroying the cell membrane, but most cells will still die because the virus has taken over its cellular activity, leaving it unable to carry out the processes necessary to live.
How your body fights back
Everyone has had a viral infection at some point in life. Some viruses can lead to chronic infections but most can be defeated with the right strategy. If a virus manages to make it into your body it meets with a formidable defense force: The immune system.
Macrophages are a type of white blood cell that can destroy germs outright. Sometimes macrophages are enough to do the job, but if they aren’t then B and T lymphocytes get called into play. B cells create antibodies, specialized proteins that bind to viruses and stop them from replicating and highlight them for other white blood cells to destroy.
T lymphocytes have a variety of different skills. They can help boost the functioning of other white blood cells and some destroy infected cells. “Memory T cells” are an exceptionally important variety. When your body encounters a new infection, it produces many white blood cells coded specifically to fight that infection. After the infection is over, memory T cells remain. These cells still are specialized to fight the one specific germ and remain active in the immune system long after the original infection is defeated. If some day in the future the original germ gets into your body again, the memory T cells will recognize it and rapidly multiply to swiftly fight off the new infection. This is why if you are exposed to a virus once, you won’t get as sick the second time.
Vaccines are extremely important because they simulate an infection without actually making you sick. They are a way of building up memory T cells in your body so they are ready in case you ever encounter the virus for real. If you get the polio vaccine, your body remembers how to fight the polio virus without you ever being in danger.
What makes viruses more potent in winter?
Experts agree that winter is the worst time for viruses. Why is it that we and our loved ones often spend so much time sniffling From October through March? Not even frequent hand washing and flu shots can protect us all the time. Research has pointed to a variety of different causes for winter illness misery.
One problem is that our immune system isn’t as effective in winter. Lower internal body temperatures correlate with lower levels of immune response. Cold weather benefits viruses directly too. The cold virus replicates faster under cold temperatures. Also, viruses naturally break down more quickly in warm weather. When cold, they can survive for a longer time on surfaces or in the air.
Human behavior helps viruses even more. People tend to congregate inside during winter and end up in close contact. With the heat on inside buildings, the air dries out. Infected water droplets are able to hang in the air better, leaving them more chance to be breathed in by their next victim.
Fortunately for us, the groundhog predicts an early spring. Maybe we’ll be able to leave our virus-contaminated indoor spaces soon! Though for those of us with allergies, it might not be much of an improvement...
- Kate Dzikiewicz, Paul Griswold Howes Fellow