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Are Viruses Alive? Viruses all are infectious particles that consist of a DNA or an RNA molecule packaged in a protein capsid, a protective coat that allows their transfer from one cell to another. Viruses infect host cells and use the host for their reproduction and metabolism. Viruses exist in two distinct states. When not in contact with a host cell, the virus remains entirely dormant. During this time there are no internal biological activities occurring within the virus, and in essence the virus is no more than a static organic particle. In this simple, clearly non-living state viruses are referred to as 'virions'. Virions can remain in this dormant state for extended periods of time, waiting patiently to come into contact with the appropriate host. When the virion comes into contact with the appropriate host, it becomes active and is then referred to as a virus. Viruses may exhibit characteristics of both living and non-living creatures depending on the definition of life used. According to Schwann and Schleiden’s “cell theory,” all living creatures are made of “individual units of life” called cells – small membrane-bounded compartments filled with a concentrated aqueous solution of chemicals. The simplest life forms are unicellular organisms; higher organisms, such as ourselves, are like cellular cities in which specialized functions are performed by different groups of cells linked by intricate communication systems. Under this definition, viruses are acellular particles and thus are definitely not alive. If one’s definition of life is a more evolutionary one, with an organism being defined as “the unit element of a continuous lineage with an individual evolutionary history” (Luria et al., 1978), then viruses are definitely alive. When one examines the criteria for a more functional and generally accepted definition of living organisms: the ability to reproduce, obtain and use energy, respond to the environment, grow, develop, and die, we find viruses are respectably somewhere in the middle. Obviously, one cannot say that viruses grow, develop or die. Viruses cannot replicate on their own, but they can do so rather efficiently inside a host cell, whose genetic mechanisms the virus subverts for its own reproduction. Viruses do not have their own metabolism, but they can hijack cellular metabolic pathways to their advantage, often times redirecting all of the virus-infected cell energy to virus production. In addition to having the ability to affect their hosts’ behavior rather profoundly, viruses can respond to their environment by means of rapid genetic mutations. by Joana Loureiro.