Virus-Host Interaction
In a typical infectious disease, the three main factors are host, agent and environment. The vector acts like a transporter in between all the three factors.
The general virus replication cycle for animal viruses involves 6 stages. The attachment, penetration, uncoating, replication and expression, maturation and release.
http://www.jbc.org/content/281/13/8305.full.pdf
Viruses have two types of replication cycle. The lytic and lysogenic cycles. Viruses that undergo lytic cycle will replicate and released out of the infected cell and infect other healthy cells. However, viruses that undergoes the lysogenic cycle will replicates but may or may not be released out of the infected cell.
Taken from Mr William How lecture notes: Virus-Host Interaction
In the attachment stage for the animal virus, the attachment sites of the virus, which are typically made up of proteins, bind to the receptor on the host cells. The receptor of the host cells is typically made of maybe protein, glycoprotein or glycolipid and they are specific during attachment.
In the second stage of the replication cycle is the penetration stage. There are three ways of penetration that occurs in the eukaryotic cells. They are the receptor-mediated endocytosis, clathrin-mediated endocytosis and fusion. Receptor-mediated endocytosis is that the virus will binds with a receptor before entering the host cell. The clathrin-mediated endocytosis is also known as the naked virus endocytosis is that the virus without its envelope will enter the host cell by endocytosis. Fusion is the direct penetration of the enveloped virus into the host cell.
The third stage is the uncoating. Uncoating is the separation of the nucleic acid from its protein coat. It is the uncoating of the genetic material of the virus. In the fourth stage, the virus will express its genetic material (i.e. translation) and replicate the genetic material.
The fifth stage of the replication cycle is the maturation. In maturation, the protein caspid is assembled.
The last stage is the release of the virus. For enveloped viruses, the release of the virus is via budding. (i.e. a lipid envelope is formed around the caspid.) For non-enveloped virus, the release of the virus is via membrane rupture. (i.e. the membrane of the host cell burst)
In the virus growth curve, in particular to the latent stage, there is a fall of concentration of virus as there are no virus particles during replication of the virus. Also the virus is unable to be detected in the external medium unless the virus is released. The new replicated virus is assembled and released out of the host cell to infect other cells when the host cell burst.
Normal microflora (i.e. microbes) can be found in the surface tissues such as the skin, eyes and mouth, of a healthy individual but not in the internal tissues such as blood, brain and muscle.
Between the virus and the human, when one organism benefit while the other is unaffected, it is called commensalism. When both organisms benefit, it is called mutualism. When one organism benefit while the other is harmed, it is called parasitism.
Taken from Mr William How lecture notes: Virus-Host Interaction
There are 4 major portals of entry for virus to infect humans. They are the skin, mucous membrane, placenta and the parenteral route. Although the skin is a barrier to most pathogen, some can still enter via hair follicles, sweat glands, cuts and bruises.
The mucous membranes are the respiratory, gastrointestinal, urinary, reproductive and the eyes. They are thin, moist, warm and cells are living. Pathogens can cross the placenta and infect the foetus. The pathogens enters the host via parenteral route such as nail and needle punctures, bites, cuts, stab wounds and surgery.
The virus that enters into a host will also leave the host to infect other people. The portals of exit are generally same as the portal of entry. The pathogens either leave the host via secretion or excretion.
The pathogenicity is the ability of microbes to cause disease. The degree of the pathogenicity is the virulence. Some of the virulence factors are the adhesion, host evasion, latency and high mutability.
There are a few stages involved in the infectious disease in a host. They are the incubation, prodormal, illness, decline and covalescence. In the iceberg concept of infection, severe symptoms of an infection are surfaced. Situations such as mild symptoms, infection with no symptoms and exposure with no infection, are not surfaced out.
Viruses that leaves the human body (host) cannot survive long, thus the places where the viruses are maintained is called the reservoirs of infection. The three types of reservoirs are animal, human and non-living.
Diseases that are spread from animal hosts to human host are zoonotic diseases. Human host can be infected from direct contact with animals and its faeces, consuming animals and via vectors such as mosquitoes and fleas.
Human host can be infected via the human reservoirs such as infectious diseases that are contagious or via sharing of food (i.e. saliva) or sneezing and coughing.
Examples of non-living reservoirs are the soil, water and food. Virus that is in the food or water (contaminated food and water) is ingested into the human host. Thus, humans are infected with the infectious diseases.
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