(1) Commonly referred to as "AMS". The mildest and most common form of altitude sickness. It develops when a too-quick ascent outpaces the body's ability to adjust.
Uncommon among hikers and backpackers, it normally happens only well above 5000 feet (1524 m), and does show up as shortness of breath, dizziness, lack of appetite, swollen extremities, insomnia, fatigue, headache, nausea, and other flu-like symptoms.
AMS's cause is unknown but may be due to swollen brain tissue starved of oxygen.
The best way to avoid mountain sickness is to take it easy, drink plenty of water, and allow the body to adjust through a slow ascent.
(2) Psychological disorder arising after months on the trail.
One manifestation is a crazy-sick desire to eat pizza, drink beer, see a movie, and sleep between sheets until well after sunrise.
Another is a crazy-sick desire to remain on the trail permanently, drifting north and south with the seasons, sleeping in vacant lots or under picnic tables, bathing in streams, eating dinner out of plastic bags, and admiring every sunset that happens.
Each type of disorder may begin subtly, but then typically advances rapidly into an acute phase which occasionally becomes permanent.
Some hikers return home and never again set foot on any trail while others disappear forever into the woods, grow increasingly hairy, and eventually lose the power of speech, though they can still whip up supper over even the tiniest of stoves, and seem to be aware of what they've become, and like it.
We few, we grumpy few, we rumply-hat geezers say to you Effort or Eff it. No sniveling.
Source: How to talk in the woods.