Version 1: Spell it out: H-A-Y F-E-V-E-R
Version 2: Sign ALLERGIC
Version 3: Spell "H-A-Y" and sign "TEMPERATURE."
(Note: Because this is a compound sign the sign "temperature" tends to be done with an abbreviated movement. ("Down, up" -- instead of down, up, down. But seriously folks this is going to vary. Don't get into arguments with anyone over this. Life is too short, and there's like a thousand versions of this dang sign anyway. Other than sign language students and anal-retentive ASL teachers -- who cares about being "right" about "one version" of "hay fever?") Also, I've noticed that a lot (um, ahem, most actually) of my friends do "temperature" with both hands in an "index" shape (not the flat shape you see above). Bah. My version is "okay" but yah, the evolutionary tendency of two-handed signs are for the handshapes to become more similar.
Version 4: Sign GRASS then sign FEVER using the back of your dominant flat-hand to the forehead.
Notes: