Flu like symptoms gone in 24 hours 4 times in last 6 months

My husband develops body aches, then over the course of day it is accompanied by a headache, mild nausea, midgrade fever and cold sweats along with exhaustion. Just when we think it is the flu, he will awake the next day slightly dehyrdrated and by noon be almost back to normal.  This has happened 4 of the last 6 months.  What is this?  He is 41 and in generally good health.