What is the difference between influenza and hay fever?

Very different: Influenza is a infectious viral respiratory illness associated with cough, fever, muscle aches, etc. Hay fever refers to allergies which are not due to an infection.

Not always: Allergies don't cause a fever, and usually last longer than a cold(10days)..An itchy nose is more frequent with allergies, and the mucous from an allergy is usually clear...Severe aches = cold not an allergy..Check out this link http://www.Niaid.Nih.Gov/topics/allergicdiseases/documents/coldallergy.Pdf.