An Ague

1. (obsolete) An acute fever.
2. (pathology) An intermittent fever, attended by alternate cold and hot fits.
3. A chill, or state of shaking, as with cold.
4. (obsolete) Malaria.

The pronunciation /ˈeɪɡ/ is a common pronunciation by people to whom this is a book word (a word one learns by reading and has never heard spoken). /ˈeɪ.ɡju/ is the standard pronunciation.