Viruses are tiny agents that spread infections and can duplicate repetitively inside the cells of living things. According to Roulston etal. (1999) “their size is so small that they cannot be viewed using an ordinary microscope.” Their damage is extensive as it can infect all kinds of organisms. Presently, scientists have discovered more than thirty thousand types of viruses that attack organisms, though this is not the exact figure as many more are being discovered. Before we go further to find out about viruses, let us look at the characteristics of living things.
Characteristics of living things
In a natural environment, living things are complex as compared to objects that have no life. They contain groups of cells that make up tissues which in turn group to form organs. Living things experience metabolic reactions. Within a living thing, there are chemical reactions that require the use of energy as well as release of energy. Living things have the ability to respond to specific conditions in the environment. It could be physical contact, heating or response to light energy. When an organism takes in substances from the environment, it builds inner structures hence resulting to growth. Through the process of reproduction, living things have the ability to reproduce. Another characteristic of living things is the ability to suit to the changing environment through the process of evolution. Lastly Rohwer argued that, “the ecological characteristic of a place has the ability to influence the life of a living thing.”
Structure and characteristics of viruses, viroids and prions
According to Rosen (2004), “the structure of a virus varies with some taking the shape of a crystal, a sphere, a rod but maintaining wings referred to as the antigens. A characteristic that makes viruses behave like living things is its ability to reproduce at a very fast rate.” On the contrary, they do not do it on their own but depend on the hosting organism. Unlike the living organisms, viruses only have one of the nucleic acids, either RNA or DNA. Viroids are virus that contain naked RNA and cause infectious diseases in plants while prions are a group of viruses that are carried in proteins and cause diseases such as mad cow and chronic wasting.
Types of viruses
Sanjuán (2010) classified viruses according to the following categories, “double stranded DNA or RNA viruses, single stranded DNA or RNA viruses, double stranded reverse transcribing or transcribing viruses and positive or negative single stranded RNA viruses.” Another method of identifying the type of virus is through the organisms that it affects with some attacking animals, plants and bacteria.
Viruses cause many diseases in the world that have been a threat to the survival of human beings. Some of these diseases include; smallpox, Spanish flu, HIV and Aids, ebola and cancer. Viroids cause diseases in plants with the following symptoms; retardation of growth, malfunctioning of leaves and drying up of plants. Prions cause diseases like the mad cow and chronic wasting.
Life is a feature that differentiates objects that can sustain themselves, that is the living things and those that cannot survive on their own, implying the nonliving things, whether those features have stopped working or the objects do not have them.
According to me, viruses, prions and viroids are not living things because they do not exhibit all the characteristics that living things that include metabolism, growth and complexity in their structure.
Rohwer F., 1997. Viral metagenomics. New Yolk: New Yolk University Press.
Rosen, F.S., 2004. Isolation of Poliovirus. New England Journal of Medicine 351 (15): 1481–83.
Roulston A, Marcellus RC, Branton P.E., 1999. Viruses and apoptosis. Oxford: Oxford University Press.
Sanjuán R., 2010. “Viral mutation rates”. Journal of Virology 84 (19): 9733–48.