What a sad look in your eyes… you, the boy in a red cap. You must be homesick. That’s nothing you need to be ashamed of. Anybody who is on a long trip will miss home. In this case, the best thing to do is to call home and hear your mom’s voice.
— Doctor • Earthbound

Homesickness is a status ailment in Earthbound. It is exclusive to Ness, who becomes homesick if he doesn't make contact with his mother for a long amount of time. It causes Ness to miss turns in-battle, during which he spends his time reminiscing about home. 

It can be cured either by holding a Chick, or, preferably, calling or visiting his mother. Note that eventually Ness will become homesick again if Ness does not maintain contact with his mother for a long period of time.

Homesickness is similar to the Asthma condition in that it is an exclusive status ailment that affects only one particular character, Ness and Ninten, respectively.