Delirium of high fevers, is caused by brainwaves of theta frequency, which are the same ones that your brain generates when in deep sleep, or just falling asleep.
This causes you to have the same experiences you might have in deep sleep, when you are completely awake, though suffering from fever.
From my personal experience i can tell you that the delirium often manifests in things like distortions in object sizes, distortions in distances, inability to command your limbs, inability to express yourself verbally, general hallucinations, feeling of your mouth being stuffed full with a gum-like substance (especially when chewing bubblegum), feelings of levitation or astral projection, paranoia, panic, feelings of falling rapidly (this is very common when you are falling asleep).
If you were to obtain a high fever in order to get high, i would tell you to go ahead, it can be worth it (not to mention you don't have to go to work/school). The thing to remember, is that you can control these hallucinations. If you just throw yourself at the mercy of the hallucinations and go with the flow, you're going to have a 'bad trip', instead try to make the hallucinations work for you, in order to get to a waking state of lucid dreaming.
This method is one of the most effective ones featured in The List of 250 Natural Highs.