Short answers, pulled from the story.
Research has suggested that the average person can distinguish over one trillion unique odors, and some individuals may be capable of distinguishing up to a thousand trillion odorants. Even the worst performer in one study could likely distinguish 80 million scents. These findings were challenged in November 2014 by Caltech scientist Markus Meister, who argued the claims rested on errors of mathematical logic.
Linda B. Buck and Richard Axel were awarded the Nobel Prize in 2004 for cloning olfactory receptor proteins and pairing odor molecules to specific receptor proteins. Their work showed that each odor receptor molecule recognizes only a particular molecular feature or class of odor molecules.
Retronasal smell plays the biggest role in flavor perception. During chewing, the tongue releases odorants that enter the nasal cavity on exhalation, creating the sensation that the smell originates inside the mouth. The human tongue can distinguish only five distinct taste qualities, while the nose can distinguish among hundreds of substances even in minute quantities.
Bloodhounds have noses ten to one hundred million times more sensitive than a human's and were bred specifically to track humans. They can detect a scent trail several days old. Grizzly bears surpass even bloodhounds, with a sense of smell seven times stronger, allowing them to detect food from up to eighteen miles away.
About 50% of patients with SARS-CoV-2 (causing COVID-19) experience some type of smell disorder, including anosmia (complete loss of smell) and parosmia (distorted smell perception). SARS-CoV-1, MERS-CoV, and influenza can also disrupt olfaction by infecting the olfactory epithelium.
The olfactory system is the only human sense that bypasses the thalamus and connects directly to the forebrain. Odor information projects to the entorhinal cortex, which connects to the hippocampus and amygdala, brain areas long associated with memory and emotion. This direct anatomical link to the limbic system is thought to explain the strong memory-triggering power of smell.