It is cold and flu season so many of us are currently under the weather with a virus. But what exactly is a virus? And are they even alive?
Outside a host cell, these weird microscopic particles, or virions, only consist of a tiny piece of genetic information (about 10,000 times less than that contained in the human genome) and a protein or lipid (fatty molecule) shell. Whether these particles are living things is the subject of much debate, as they don’t meet many of the usual criteria for life.
While there isn’t any formal agreement on what defines life, most definitions include the ability to adapt to the environment, to reproduce, to respond to stimuli, and to use energy.
While the virus particle may fall short of the definition of life depending on the criteria used, for some virologists like myself, thinking of the virion as the “virus” is like calling a sperm or unfertilised egg a “person”. Sure, a sperm is an essential step towards creating a person, but few people would argue that a sperm or unfertilised egg should be described as the finished product.
Much like a sperm, virions are produced in the millions. Many will never reach their destination and are lost and degrade in the environment. It is only when the virus binds to and enters a target cell that its cycle of replication can begin.
A virion doesn’t even always contain a majority of the molecules a virus can create. For example, the norovirus virion contains just three different types of protein and one type of RNA (a nucleic acid like DNA which uses a different sugar to form its backbone). Infected cells, however, make at least eight different viral proteins and four different viral RNAs.
Nor does the virus particle itself usually result in the symptoms of disease. Typically, when you catch a virus, your symptoms come from either infected cells dying, or your immune response to those infected cells.
For these reasons, some virologists consider the infected cell, rather than the virion, to be the virus.
I am virus
While this idea sounds outlandish, from conception to grave, your cells are intricately associated with viruses. Even if you don’t have a cold or the flu, you are still part-virus as human DNA plays host to a range of different viruses.
These are retroviruses, the best-known example of which is HIV. While HIV only entered the human population relatively recently, viruses very much like it have been infecting us and the creatures we evolved from since long before humans even existed.
While HIV infects immune cells, when a retrovirus instead infects the cells that produce eggs or sperm, the viral DNA can be inherited by any offspring. Over millions of years, these viruses have lost their ability to produce infectious particles, but have in some cases found other vital roles, and are now indispensable for human life.
One well-studied example is a protein called Syncytin-1, which is vital for the development of the placenta. This was originally a retroviral protein which entered the monkey population which gave rise to humans around 24m years ago. If we deleted this protein from our DNA, humanity would rapidly go extinct as we could no longer produce a functional placenta.
All these viruses which inserted into our DNA long ago are termed endogenous retroviruses (ERVs). In humans, ERVs have long since lost the ability to produce infectious virions, but this is not the case in all animals. Pig ERVs, for example, can produce infectious particles and are a concern when considering the use of pig organs for transplant, as these are known to be able to infect human cells in the lab.
If a virus is the infected cell, rather than the virion, you could even think of the viruses that can infect us as more than 99.9% human. This is because they need many of the human proteins or other molecules present in your cells and encoded in your DNA to make more virus.
A human cell is vastly more complex than even the largest virus, and viruses can make use of this to compensate for their own simplicity. Viruses and their host cells share many common needs. They need to be able to produce RNA, protein, lipids and have access to the raw materials to generate these. As a host cell already contains all the needed components to achieve this, a virus can simply provide its own instructions, in the form of the viral genome, and let the cell do most of the work.
It takes many more cellular proteins to make a virus, than it does viral proteins. A virus only needs to provide instructions for the few components the host cell cannot produce. An example of this would be viruses which have a virion with a lipid membrane, such as influenza. This membrane is usually recycled from host cell membranes. The addition of a couple of viral proteins converts this into the membrane coat of the virion.
This use of host components by viruses also makes it clear why it has been so difficult to develop effective antiviral drugs. Much as with cancer treatment, there is very little to distinguish infected cells from normal human cells, which makes coming up with a drug that will only target infected cells extremely challenging. To be effective, you have to target that tiny part of the infected cell that is purely virus, without harming the remainder.
So are viruses alive? It’s still not settled, and really depends on what you think a virus is. What does seem clear, however, is that the viruses which infect us can be seen as part human, and we are part virus.