Symmetrel is primarily used to treat which condition?

Symmetrel, or amantadine, is primarily used to treat influenza, particularly for type A influenza. It functions as an antiviral medication, working to inhibit the replication of the virus within the body and thus alleviate the symptoms associated with the flu. Additionally, it has approved uses for treating certain movement disorders like Parkinson's disease, but its main application in the context of this question is in managing influenza.

The other options do not accurately represent the primary therapeutic use of Symmetrel. It is not indicated for treating cough directly, as cough can be a symptom of various conditions, including influenza itself. Similarly, while it may impact fever indirectly by treating influenza, it is not used specifically to reduce fever. Lastly, it is not indicated for alleviating allergy symptoms; those are treated with antihistamines or other allergy-specific medications. Therefore, its designation as an antiviral for influenza is the most relevant and accurate answer.