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.