Viral Entry and Spread

Viral Entry

3 requirements to be fulfilled to ensure successful infection in an individual host:

• Sufficient virus must be available to initiate infection


• Cells at the site of infection must be accessible, susceptible and permissive for the virus


• Local host anti-viral defense systems must be absent or initially ineffective
  

To infect its host, a virus must first enter cells at a body surface. Common sites of entry include:

- Mucosal linings of the respiratory
- Alimentary tract
- Urogenital tract
- Outer surface of the eye (conjunctival membranes or cornea)
- Skin
* Only the two points in purple will be covered.

Alimentary Tract - a common route of infection and dispersal.

Eating, drinking, and some social activities routinely place viruses in the alimentary tract. It is designed to mix, digest and absorb food, providing a good opportunity for viruses to encounter a susceptible cell and to interact with cells of the circulatory, lymphatic, and immune systems.

Urogenital Tract

Some viruses enter the urogenital tract as a result of sexual contact. The urogenital tract is well
protected by physical barriers, including mucus and low pH (in the case of the vagina). Normal sexual activity can result in minute tears or abrasions in the vaginal epithelium or the urethra, allowing viruses to enter. Some viruses infect the epithelium and produce local lesions (e.g., certain human papillomaviruses, which cause genital warts). Other viruses gain access to cells in the underlying tissues and infect cells of the immune system (e.g., HIV type 1), or sensory and autonomic neurons (in the case of herpes simplex viruses).

Viral Spread

* Click on the picture for enlargement!!

Systemic infection, which was not mentioned in the diagram, is one that affects the whole body, probably travelling in lymph or blood. This is in contrast to a local infection which only affects the area where the infection entered.