With the recent Ebola outbreak in Africa, I’ve had a number of individuals question me as to what exactly a virus is. When I ask them what they think it is, they usually say a very small organism related to a bacteria. In actuality viruses are quite a bit different from bacteria and trying to understand the difference can be quite a task for those without a biology background. In this post I’m going to walk you through it by placing viruses in the larger context of life on Earth.
Most people are familiar with cells. These are the living building blocks of plants and animals. Cells are self-contained structures (due to their outer cell membrane). They take in nutrients from their surroundings and convert these into energy, store hereditary material (DNA), and are capable of duplicating themselves. The human body has trillions of cells, all working together to provide complex function and the ability for us to heal when injured. Cells can exists independently of each other and some of the smallest organisms are just that – single cells that have their own lives. They take in nutrients, create energy, and divide, creating more of themselves. The figure above and to the left shows an example of a single-celled bacteria. Bacteria are everywhere and we know that some can be very beneficial (the Escherichia coli in your gut) while others can pose health hazards (Bacillus anthracis which causes Anthrax).
It’s relatively easy to view bacteria as being “alive” since they, all by themselves, take in nutrients, convert nutrients to energy, store heritable material like DNA, and reproduce. Bacteria also make sense in the larger spectrum of life as they help the decomposition of matter (dead animals, plants, etc.) and they are also food for larger single-celled organisms and small crustaceans. These organisms are, in turn, food for larger organisms and the trend continues all the way up to humans which consume a wide variety of plants and animals. Bacteria are quite small (between 0.5 and 5 micrometers). Viruses, on the other hand, are 0.02 – 0.4 micrometers in size. So, viruses are generally smaller than bacteria and it’s natural to assume they fall below bacteria on the “food chain.” Perhaps bacteria feed on viruses and so on. However, the reality is somewhat different.
Viruses are not cells. They consist of heritable material (DNA or RNA) surrounded by a protein coat, which is in turn surrounded by an envelope of lipids (fats). A typical virus can be seen in the picture to the right. Left alone viruses cannot take in nutrients from their surroundings and convert them to energy, nor can they reproduce themselves. The gut reaction when people first hear this is…viruses aren’t alive? It’s actually generally accepted that viruses are alive because once they infect a host’s cells (or the host cell if the host is just one cell like a bacteria), they use the cell’s machinery (organelles) to generate energy and reproduce. Viruses can infect bacteria, plants, animals. Nothing is immune. A virus infects a cell and highjacks the the cell’s normal infrastructure for living in order to support a rapid reproduction of itself, which then results in the destruction of the host cell. For single-celled organisms, this kills them, for multicelled organisms like humans, it causes cell death in our bodies which we can sometimes recover from (a Cold) and other times have a very hard time recovering from (Ebola).
From an evolutionary perspective viruses are very confusing. We don’t really know how they arose. We also don’t really understand their “purpose.” It’s much easier to see the purpose of almost every other organism as they tend to do something constructive like generate oxygen, breakdown decaying matter, or at least, they are food for other organisms. You might look at humans and think, well we are somewhat of an exception as well. We don’t really have any predators and can certainly do more harm than good to an ecosystem if we aren’t careful. For a “circle of life” view you might think of viruses preying on humans, as they can certainly kill us, but it’s still a bit forced as viruses infect everything, not just humans, and nothing really preys on viruses to complete the cirlce. So, even today, viruses are still a bit of a mystery. They are exquisitely designed to infect other organisms and propagate themselves at extremely high rates/numbers. We know much more about viruses today compared to when they first came across our radar in the late 1800s, but there is clearly still much more to understand.