The upper respiratory tract includes the sinuses, nasal passages, pharynx, and larynx. These structures direct the air we breathe from the outside to the trachea and eventually to the lungs for respiration to take place.

Infection of the specific areas of the upper respiratory tract, include rhinitis (inflammation of the nasal cavity), sinus infection (sinusitis) inflammation of the sinuses located around the nose, common cold (nasopharyngitis), inflammation of the nares, pharynx, hypopharynx, uvula, and tonsils, pharyngitis (inflammation of the pharynx, uvula, and tonsils), epiglottitis (inflammation of the upper portion of the larynx or the epiglottis), laryngitis (inflammation of the larynx), laryngotracheitis (inflammation of the larynx and the trachea), and tracheitis (inflammation of the trachea).

Upper respiratory infections are one of the most frequent complaints with varying symptoms ranging from runny nose, sore throat, cough, to breathing difficulty, and lethargy.

Although upper respiratory infections can happen at any time, they are most common in cold weather conditions. This may be explained because these are the usual school months when children and adolescents spend a lot of time in groups and inside closed doors. Furthermore, many viruses of the upper respiratory infection thrive in the low humidity of the winter.

Majority of upper respiratory infections are due to self-limited viral infections of the upper respiratory tract. 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.

Common symptoms of the upper respiratory infection generally include: nasal congestion, runny nose (rhinorrhea), nasal discharge (may change from clear to white to green), nasal  breathing, sneezing, sore or scratchy throat, painful swallowing (odynophagia), cough (from fever laryngeal swelling and post nasal drip). Commonest symptoms are generalized body weakness and (more common in children).

Some common risk factors for upper respiratory infection include physical or close contact with someone with upper respiratory infection, poor hand washing after contact with an individual with upper respiratory infection, close contact with children in a group setting, schools or daycare centers, contact with groups of individuals in a closed setting, such as, traveling, tours, cruises, smoking or second-hand smoking (may impair mucosal resistance and destroy the cilia), health care facilities, hospitals, nursing homes. Others include immunocompromised state (compromised immune system) such as, HIV, organ transplant, congenital immune defects, long term steroid use and people with anatomical abnormalities as in facial trauma, upper airway trauma, nasal polyps.

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