None of the traditional plagues have been spread by air over long distances.

Some epidemics (e.g. influenza, chickenpox or smallpox, TB) are spread by airborne respiratory droplets over SHORT distances, but that is considered host to host spread.  Others are spread by fecally-contaminated WATER in inadequate sanitary conditions (cholera, dysentery, typhoid fever) or by insect vectors (malaria, plague, epidemic typhus, various encephalitides). 

However, there *is* a fungus, Coccidioides immitis, endemic to the lower Sonoran desert zones (e.g., the California Central Valley, parts of Arizona, Mexico, and Central & South America), which is spread by airborne spores.  Uncommonly, it can be blown long distances in dust storms and cause disease outside of its endemic area, but that has generally only been responsible for a few hundred cases at most.  After inhalation, coccidioidomycosis (known as Valley Fever from the San Joaquin Valley) causes a respiratory syndrome, which is usually where it stops.  However, if a very large number of spores are inhaled, or if the host is immunocompromised, the disease can spread systemically (especially to the lining of the brain, bones/joints, and skin), and be fatal.  But again, this has NOT been a cause of widespread epidemics. 

I suppose that if such a fungus were magically mutated to overcome the usual host defenses, or if the soil conditions in Mordor were changed to favor the growth of an extraordinarily large number of such spores (it likes to live in alkaline soil with a high concentration of calcium sulfate or calcium borate), it is conceivable that the plague could have been "Valley Fever".