Upper respiratory tract infection

An upper respiratory tract infection is any infection that involves the nasal cavity, paranasal sinuses, pharynx, or larynx, and it’s most often caused by an invading pathogen like a virus.

When you breathe in, air flows through the nostrils and enters the nasal cavity, which is lined by cells that release mucus.

That mucus is salty, sticky, and contains lysozymes, which are enzymes that help kill bacteria.

Nose hairs at the entrance of the nasal cavity get coated with that mucus and are able to trap large particles of dust and pollen as well as bacteria, forming tiny clumps of boogers.

The nasal cavity is connected to four sinuses which are air- filled spaces inside the bones that surround the nose, there’s the frontal, ethmoid, sphenoid, and maxillary sinus.

The paranasal sinuses help the inspired air to circulate for a bit so it has time to get warm and moist.

The paranasal sinuses also act like tiny echo-chambers that help amplify the sound of your voice, which is why you sound so different when they’re clogged with mucus during a cold!

So the relatively clean, warm, and moist air goes from the nasal cavity into the pharynx or throat.

At each side of the back of the throat, there is the pair of tonsils, which are small are clumps of lymphoid tissue that act as the body's first line of defense that swallow viruses and bacteria that enter through the mouth or nose.

The lower part of the pharynx is continuous with the larynx or the voice box.

Αt the top of the larynx sits a spoon- shaped flap of cartilage called the epiglottis which acts like a lid that seals the airway off when you’re eating, so that the food can only go one way - down the esophagus and towards the stomach.

Now, once air makes its way into the larynx, it can continue its journey through the trachea, or windpipe, towards the lungs.

Now, in addition to air, you’re constantly breathing in other stuff like viruses or bacteria.

For example, when an infected person sneezes or coughs, they spread thousands of droplets containing these pathogens into the local area, up to about two meters or six and a half feet away.

These droplets can then land in the mouths or noses of people nearby, or get inhaled into the upper airways.

Most of these viruses or bacteria can also survive on surfaces for a few hours, so it’s possible to get them by touching a surface, like a contaminated doorknob, and then touching your own eyes, nose, or mouth.

Usually, even when a pathogen gets in, we’re good at protecting ourselves, but sometimes, a particularly nasty pathogen succeeds in colonizing our upper airways and when that happens - Congratulations! You’ve got an infection!

These infectious little pathogens typically jump inside the cells lining the airways, multiply and cross over to the underlying tissue, creating an inflammatory response.

When that happens, goblet cells and submucosal glands in the airways start to produce a lot of mucus in order to try to trap and eventually expel these pathogens.

In severe situations, the pathogens might result in lots of white blood cells coming over to fight off these pathogens.

The battle ensues with the result being pus - a mixture of pathogens, immune cells, and dead tissue and a whole lot of inflammatory signaling molecules, called cytokines.

These cytokines can then spill into the systemic circulation and reach the brain, telling it to rise the body’s temperature in order to make it a less friendly place for those pathogens to reproduce.

Alright now, there are many different types of upper respiratory tract infections, depending on the part of the tract that’s involved.

In rhinitis, “rhino-“ means nose, so the infection is inside the nasal cavity.

Usually it’s caused by viruses responsible for the common cold or flu, and the most common is rhinovirus, influenza virus, respiratory syncytial virus- or RSV for short, parainfluenza virus, and adenovirus.