Viruses: Discovery of viruses and how they work

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Viruses
Introduction
Viruses are non-cellular, sub-microscopic, intracellular parasites that survive by attacking a cell and replicating quickly within the cell, thereby resulting in the death of the cell. They are usually proteinous in nature. Viruses are microscopic organisms that cannot reproduce or thrive outside a host. Throughout history, viruses have been the source of several epidemics, including SARs, Zika, Ebola, and MERS. All these viruses have brought with them serious impacts on the global population. They have historically been the major causes of deaths on a large scale. Today, we are witnessing the impact of SARS-CoV-2, which causes COVID-19. Other than human beings, viruses infect other types of life forms, including plants, archaea, bacteria, and other microorganisms. Despite being synonymous with infections, viruses are important research tools and play an important role in human and animal health.
Discovery of viruses and how they work
The discovery of viruses dates back to 1886 when Adolf Mayer, a German scientist, realized that healthy tobacco leaves discolor when they are injected with noxious juice from infected leaves. Although Mayer identified that whatever was causing tobacco mosaic disease was in the leafy juice, he was unable to isolate or recognize the disease-causing agent. In 1898, Martinus Beijerinck, a Dutch scientist, suggested that the cause of the tobacco mosaic disease was a living liquid virus known as the filterable virus. In 1939, the discovery of viruses became full cycle after "Ernst Ruska and Max Knoll visualized the tobacco mosaic virus using a high-resolution electron microscope" (Kruger 1713). Viruses usually infect a susceptible cell then direct the cell to produce more viruses. Generally, viruses cannot thrive independently because they lack the ability of the host's cells.
Viruses usually attack a cell, then utilize the energy supplied by the cell to replicate. They take up the operations or activities of the cell by mimicking its functionalities. After replicating for a while, the cell explodes, thereby dying. The new viruses then move onto the next cell and repeat the same process. In the end, what the individual might experience can range from mild health problems to serious effects that might even result in death. As of 2012, there were about 219 known virus strains in the world (Woolhouse, Scott, Hudson, Howey, and Chase-Topping). That figure has obviously been surpassed by now, as there are new discoveries made on a regular basis.
Structure and types of viruses
The structure of viruses includes an inner nucleic acid surrounded by an outer casing known as the capsid. Capsids protect viruses from being chewed or destroyed by the cells of the hosts. The DNA or RNA found in the core of viruses can be either single-stranded or double-stranded and contain the genetic information of viruses (Singh). There are various types of viruses, and the total number of viruses is estimated to be more than the stars in the galaxy. Viruses are classified based on the shape of their capsid. For instance, in helical viruses, the capsid is made up of a single protein subunit, while in complex viruses, the capsid has extra ...