The viral replication process begins when a virus infects its host by attaching to the host cell and penetrating the cell wall or membrane. Then the viral genome hijacks the host cell's machinery, forcing it to replicate the viral genome and produce viral proteins to make new capsids. In the past, viruses were considered nonliving infectious particles, little more than genetic material wrapped in a protein capsid. Today, virologists are beginning to think of viruses as living organisms that can be classified phylogenetically into defined species, much like any other living organism. The primary reasons for this shift in attitude can be partially attributed to the discovery of giant viruses, having large genomes and complex regulatory systems. Aside from that, it has become obvious that viruses lead complex lives; they evolve, speciate, and participate in the evolution of all classes of living organisms. In this chapter, we will discuss the early attempts to classify viruses, and review the biologic properties of the classes of virus that contain human pathogens.
Constants are the factors that stay the same throughout the entire experiment no matter which group. The definition of a constant is "a situation or state of affairs that does not change." this furthers the point that constant is the correct answer because they don't change in the experiment.
