In my case, I just felt a little tired for a few days and then whammo, I was flat on my back with fever. A friend told me that it sounded like a nasty bug that had been going around which had put a friend of hers in the hospital, so I started watching my temperature, which I don't ordinarily do.
I'm glad I did it this time. My temperature bounced around a lot. At one point, I thought I was feeling a bit better --- I was tired but lucid, and able to get up and move around a little --- when my temperature started climbing precipitously. At 103° I sat in the bathtub with an ice pack on my head and started running cold water into the bath. At 103.5° I was up to my chest in very cold water. This is not fun, but less fun was watching my temperature climb to peak at 104°. It dipped back down --- to 103.5° --- but held there for a discomfortingly long time. After an hour in the tub, maybe a little more, my fever suddenly just broke completely for an hour. (Then made a comeback, but never again quite so bad. I'm pretty steady at 100.5° today.)
No harm done: 104° is actually sustainable for a little while without ill effects. But if you get up to 105°, your brains start to cook. I dread to think what might have happened had I not been watching the thermometer.
My point being: if you're feeling bad, start checking your temperature. The bug going around is a doozy.