Why do Swiss mercenaries often get sick while serving abroad

In the late 17th century,a medical student named Johannes Hofer noticed a strange illness affecting Swiss mercenaries serving abroad.Its symptoms,including fatigue,insomnia,irregular heartbeat,
indigestion,and fever were so strong,the soldiers often had to be discharged.As Hofer discovered, the cause was not some physical disturbance,but an intense yearning for their mountain homeland.
He dubbed the condition nostalgia,from the Greek ; for homecoming and for pain or longing.