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.