While viral disease can cause changes in cognition, it only usually does so when it gets REALLY bad. I don’t think you suffered from an infection. It is, however, possible that you all got poisoned with something. My guess is it was either psychogenic, or poisoning. Did everyone who got sick have any common factors? Ate at the same cafeteria, ordered the fish, etc?
hmmm…
- Peak season in national parks is July-August.
- Three-4 weeks later is…when the new semester begins.
- Most employees of the park’s concessionaire are students.
I offer no medical diagnosis, and I suspect that there is no need for one 
Most park concessionaires provide very poor working conditions*.
Many workers go through a known cycle of emotions: the early-season thrills of living in a Park turn into disillusionment as you spend so much time washing dishes that you don’t have time or energy to enjoy being there.
*(At least they did when I worked there in the 1980’s!)
Each National Park bids a contract (= “concession”), which is given to one (and only one corporation) which wins the rights to operate all the hotels, gift shops, etc in the Park. The corporation effectively reigns as an almighty, all-powerful monopoly.
Whereabouts? And what did you do?
I’m thinking dehydration.
Well I also favour mass hysteria, but the symptoms do tend to coincide with mild, prolonged neurotoxin poisoning: general malaise, inability to think clearly, fatigue, followed by muscle pain.
The major problem with it being poisoning is that everybody got exactly the same symptoms. Nobody got more severe symptoms and nobody was mildly affected. Even if it was in the water, the range of body weights and differences in consumption should have seen a spread of symptoms from barely noticeable to those with more acute symptoms such as diarrhea and stomach cramps.
The same objection can be applied to any microbial illness. In a population that large we would expect some people to be utterly immune, some to exhibit mild symptoms and some to experience severe symptoms. I can’t think of a single physical disease/syndrome where every victim experiences the exact same symptoms regardless of any other factors.