A virus is a small infectious agent that can only replicate inside cells of another organisms. Unlike other pathogens, viruses require a host cell to replicate and spread to other adjacent cells as they do not possess the ability to reproduce on their own. Because they are completely dependent on the cellular metabolism of the host for reproduction, viruses are often referred to as obligate intracellular parasites.
These non-cellular human pests typically contain one of two types of genomes, made up of either short strands of DNA or RNA. Shortly after a virus enters a cell, it is usually the synthetic machinery of the host cell that allows the virus to create specialized elements called capsid that allow their genome to transferred to other cells efficiently.
Generally, viruses are too small to be seen directly using a light microscope. The average virus is about one one-hundredth (1/100) the size of the average bacterium, and most viruses that have been studied have a diameter between 10 nm (nanometres) and 300 nm (Figure 2).
Although viruses may induce profound effects on human beings, they also infect all types of organisms, including plants and bacteria. Bacterial viruses, however, are referred to as bacteriophages. In order to survive viral infections, animal and human hosts must be able to monitor their tissues closely for viral infections and be able to respond rapidly using both innate and acquired systems. Optimal responses to viruses involve: (1) Cell-mediated responses, including cytotoxic T lymphocytes (CTL) and natural killer cells which destroy infected targets that have presented viral antigens via MHC class I molecules, and (2) antibodies, which neutralize the virus by blocking attachment to host cell receptors and facilitate antibody-dependent cell-mediated cytotoxicity (Figure 3).
Since their initial discovery in 1898, about 5,000 viruses have been described in detail, although there are millions of different types (Figure 4). The study of viruses is known as virology, a sub-specialty of microbiology.
The following list of viruses have been profiled:
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