Upper Respiratory Infection (cont.)

A majority of upper respiratory infections are due to self-limited viral infections. Occasionally, bacterial infections may cause upper respiratory infections. Most often, upper respiratory infection is contagious and can spread from person to person by inhaling respiratory droplets from coughing or sneezing. The transmission can also occur by touching the nose or mouth by hand or other object exposed to the virus.

Upper respiratory infection is generally caused by the direct invasion of the inner lining (mucosa or mucus membrane) of the upper airway by the culprit virus or bacteria. In order for the pathogens (viruses and bacteria) to invade the mucus membrane of the upper airways, they have to fight through several physical and immunologic barriers.

The hair in the lining of the nose acts as physical barrier and can potentially trap the invading organisms. Additionally, the wet mucus inside the nasal cavity can engulf the viruses and bacteria that enter the upper airways. There are also small hair-like structures (cilia) that line the trachea which constantly move any foreign invaders up towards the pharynx to be eventually swallowed into the digestive tract and into the stomach.

In addition to these intense physical barriers in the upper respiratory tract, the immune system also does its part to fight the invasion of the pathogens or microbes entering the upper airway. Adenoids and tonsils located in the upper respiratory tract are a part of the immune system that help fight infections. Through the actions of the specialized cells, antibodies, and chemicals within these lymph nodes, invading microbes are engulfed within them and are eventually destroyed.

Despite these defense processes, invading viruses and bacteria adapt various mechanisms to resist destruction. They can sometimes produce toxins to impair the body's defense system or change their shape or outer structural proteins to disguise from being recognized by the immune systems (change of antigenicity). Some bacteria may produce adhesion factors that allow them to stick to the mucus membrane and hinder their destruction.

It is also important to note that different pathogens have varying ability to overcome the body's defense system and cause infections. Some viruses may infect by much fewer numbers than others.

Furthermore, different organisms require varying time of onset from when they enter the body to when symptoms occur (incubation time). Some of the common pathogens for upper respiratory infection and their respective incubation times are the following:

• rhinoviruses, 1-5 days;
• group A streptococci, 1-5 days;
• influenza and parainfluenza viruses, 1-4 days;
• respiratory syncytial virus (RSV), 7 days;
• pertussis (whooping cough), 7-21 days;
• diphtheria,1-10 days; and
• Epstein-Barr virus (EBV), 4-6 weeks.

Source: MedicineNet.com
http://www.medicinenet.com/upper_respiratory_infection/article.htm