Colds are mostly viral infections, but can also be followed by bacterial infections, which is why there is often pus nasal discharge in the later stages. A single viral infection can completely destroy the olfactory nerve and cause permanent loss of smell, even if the nasal and sinus infections are improving. You can treat it experimentally with hormones, including nasal hormones (Reynocort or Nesuna nasal spray) and, at the same time, prednisone 30mg for 5 days, taken in the morning on an empty stomach, if there is a partial recovery, it is possible that it will slowly get better later, otherwise, the recovery is less likely.