What Viruses

Given that this chapter and the one on influenza that follows both deal with viruses, knowing exactly what a virus is and how it functions is probably a good idea. To that end, viruses are

1 Microscopic entities: The smallest microorganisms are viruses, even if not all viruses are smaller than all other organisms.

Viruses used to be considered the smallest of all microorganisms (the smallest things are still viruses) until scientists found a really big one — so big, in fact, that you can see it under a light microscope. This Baby Huey of viruses is bigger than a lot of bacteria, and it infects amoebas.

1 Obligate intercellular parasites: This is a fancy phrase that means viruses can survive and multiply only in living cells. Whose living cells? Why the hosts', of course.

A virus is an extremely simple organism. It consists of the viral genome (which can be made up of DNA or RNA) and any other molecules that the virus needs to get going after it's invaded the host cell. All this stuff is enclosed in an outer coat made of proteins or, in some cases, of proteins and lipids, which is a fancy word for fat.

Are viruses alive ? We say "Yes!"

Viruses are odd — so odd that no consensus exists on whether they're actually living things. Some people say yes; some people say no (and some people say sort of, but I'm not going to worry about the fence-sitters here).

One way to classify things as living or not living is to come up with a list of things that we think living things should have and then ask if our test object — in this case, viruses — has those things. Consider these examples of things that living organisms share:

1 Cells: With the discovery of the microscope, which predated the discovery of viruses, scientists saw that all the living things they looked at had cells, so the definition of a living thing came to include having cells. Then scientists discovered viruses, which don't have cells. So under the "all living things have cells" definition, a virus, which doesn't have cells, can't be a living thing.

ii A genome made up of nucleic acid: Viruses have a genome, the instruction set for making the organism, and when it's time to reproduce, they make new copies of the genome and package them into progeny viruses. The viral genome also codes for proteins, just as the human genome does, but because viruses are parasites, they don't need to make all the proteins — just the ones that the host doesn't already make. If having a nucleic-acid genome that codes for the production of proteins is the definition of life, viruses are alive.

If you say that, to be considered living things, viruses must have certain characteristics, then they are (or aren't) living organisms based on the characteristics you select. But this is a very nebulous way of defining what's alive and what isn't. If you say, for example, that having hair is a defining characteristic of a living organism, then you'd be alive, but a fish wouldn't. This example is silly, of course, but it shows how definitions have no foundations other than the ones humans give them.

A more helpful way to determine whether something is alive is to think about fundamental processes associated with life. Living things reproduce, for example. They also evolve. The process of copying the genome isn't error free, and mutations occur; mutations that are advantageous may increase in frequency. Being able to evolve is what fundamentally separates the living from the not living. A fire spreading through a forest reproduces itself, and a few flames lead to more flames, but fire itself doesn't evolve.

Viruses are independently evolving entities. They reproduce. They contain within themselves the instruction set for their own reproduction: the genome. And that genome changes through time: it evolves. If you use the life processes of reproduction and evolution as the fundamental characteristics of living organisms, viruses are alive.

The fact that viruses evolve is important not only for understanding where they come from and how they work, but also for addressing the threat that they pose and providing treatment. To treat a patient who's been poisoned, you just need to know how to counteract that particular poison. Treating a patient whose illness is the result of an organism that can evolve requires anticipating and responding to the adaptations that the organism can make. In the case of HIV, by the time a patient experiences symptoms, the virus has evolved and behaves differently from the original virus that infected the patient.

Whether you want to consider viruses living or not is entirely up to you. Regardless of which side of the debate you land on, all the information in this chapter still applies. Alive or not alive, viruses evolve, and that's the important thing.

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