Anosmia is the loss of the olfaction (the sense of smell). The term is also used for people who never had this sense. The loss can be temporary, or permanent. The conditions that cause a temporary loss of this sense include:
Upper respiratory tract infection, for example a common cold
Obstructing the nasal cavity, eg. through seasonal allergies, or a nasal polyp.
Side-effects of certain drugs or of certain poisons.
Concussion, brain tumors.
Olfaction depends on two nerves (Nervus olfactorius and Nervus trigeminus), as well as the part of the brain which processes the information. Damage to this system will have an effect on olfaction; permanent damage may lead to a permanent loss of olfaction.
There are tests that can be done to find the cause of the problem.
The sense of smell also affects the sense of taste. People lacking a sense of smell are limited to the basic tastes: sweet, sour, bitter, salty, and umami.
Note also that the sense of smell changes with age. Anosmia can also point to different neurological conditions such as Parkinson's disease, Alzheimer's disease, or Schizophrenia. Certain plants, such as tobacco also contain substances which change olfaction.
In general, anosmia which has its cause in the brain cannot be treated. Anosmia which is caused by obstruction can be treated by removing the obstruction. In some cases, accupuncture was successfully used to treat anosmia