The upper respiratory tract consists of the nasal passage, sinus, larynx, and pharynx. Any infection of these parts is called as an upper respiratory tract infection. These structures direct the air we breathe in, from the outside of the trachea, eventually to the lungs to respire.

Types of URTIs are:

• Common cold
• Sore throat – Pain or irritation of the throat
• Tonsilitis – Inflammation of the tonsils
• Pharyngitis- Inflammation of the pharynx
• Laryngitis – Inflammation of the larynx (voice box)
• Sinusitis – Inflammation of the sinuses
• Otitis media – An infection of the air-filled space behind the eardrum (the middle ear)
• Bronchitis – Inflammation of the bronchial tubes
• Epiglottitis – Inflammation of the upper part of the trachea

Causes and risk factors leading to upper respiratory infections
Most URTIs are caused by viruses and some by bacteria too. Though infections can happen at any time of the year they are most common in the months of September to March (fall and winter). This could be because most viruses thrive in the low humidity of winter and fall. Another reason could be that during these months more time is spent indoors in schools and offices due to the weather and thereby the infection spreads. The infection is contagious and spreads from person to person by inhaling droplets spread through coughing, sneezing or by touching the nose or mouth and spreading through the hand or handkerchief, or other objects exposed to the virus.

Barriers in the human body system to prevent infections
The infection is generally caused by the invasion of the mucous membrane by the pathogens (bacteria or virus). The mucous membrane is the inner lining of the upper airway. In order to attack and infect the mucous membrane, the virus and bacteria have to fight through various physical and immunological barriers in the upper respiratory system. The barriers start right from the nose. The hair in the lining of the nose is a physical trap for the invading organisms. The wet mucus inside the nasal cavity also engulfs the virus and bacteria that enter the upper airways. The trachea also has hair like particles which trap and move the pathogens to the pharynx, which is then swallowed into the digestive tract.

The immune system in the upper respiratory tract also acts to protect against the infection. The lymph nodes in the adenoids and tonsils in the upper respiratory track act as protectors to fight the invasion of the pathogens. The antibodies and microbes present in the lymph nodes engulf the viruses and destroy them.

Though the body has various defense mechanisms we have to think of the virus and bacteria as invaders, which somehow find a way to attack the body. They adopt various mechanisms to fight the body’s defense system and stay in the body. They produce toxins to impair the defense of the body, they change their shapes to avoid being recognized by the defense mechanism, and sometimes the bacteria produce adhesives to stick to the mucous membrane and hinder the destruction.