Viruses: a virus is a minute microorganism too small to be seen under an ordinary microscope. It is much smaller than bacteria, so tiny that they cannot even be seen through an ordinary microscope but require a special apparatus, the electron microscope. Thus, it was not discovered until sometime later than most bacteria.
Another reason they are harder to study is that, unlike bacteria, they are extremely fussy about where they grow, normally refusing to have anything to do with laboratory “food”. With the greatest of difficulty, the scientist have succeeded in working out ways of getting them to grow anywhere except in an animal body. Fussy as they are, they are extremely potent; very small numbers can start a disease. Since 1900, many diseases, such as smallpox, common cold, poliomyelitis, yellow fever, influenza, infectious hepatitis, rabies, chicken pox, measles, mumps, and other contagious diseases, have been found to be caused by filtrable viruses.
- Viruses are considered obligate intracellular parasites as viruses are cannot replicate outside of the host cell
- Virus binds to specific receptors on the host cell and then moves into the host cell. Initially, viruses evade many defense mechanisms by hiding within host cell.
- But once inside the host cell, the virus converts the host cellular metabolism to nucleic acids and proteins that are encoded and controlled by the virus.
- Viruses have the ability to either directly kill the cell or modify certain cellular functions, such as protein synthesis
- Viruses can cause cells to proliferate rapidly and randomly, causing tumors to form in the body
- The cell ultimately loses its ability to function.
- Next, the virus releases particles outside of the cell (virions), which can enter and infection other nearby cells.
- As the virus moves from cell to cell, the immune response is activated.
- Viral infections are often severe enough to spark a strong, eradicating immune response. This exaggerated immune response allows neutralization of the virus and resolution of the infection. The infected host cells are eliminated, and the virus can no longer replicate.
- In this regard, many viruses, such as those associated with the common cold, are considered self-limiting; that is, the infection ceases after a certain period of time.
- Some viruses are sustained for longer periods in the host. If the initial, acute infection does not provoke a strong viral-eradicating immune response, the individual is more likely to host a chronic viral infection (e.g. Chronic Hepatitis B).
- chronic viral infections are promoted by various viral and host factors:
- the size of the virus that is inoculate into the body
- the process of viral replication
- the viral genotype
- host susceptibility
- The virus will reside in a host cell and cause minimal or no loss of functional capabilities for that cell. However, it can cause active disease against the body in a later time.
- chronic viral infections are promoted by various viral and host factors:
- Viral infections are often severe enough to spark a strong, eradicating immune response. This exaggerated immune response allows neutralization of the virus and resolution of the infection. The infected host cells are eliminated, and the virus can no longer replicate.
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