The guy, thankfully, is much more aware of his own physical condition than I am.
When I'm sick, I think, well, I could be sick, but maybe it's just a passing thing. I'll go to work, then see how I feel. Sometimes I'm OK, but there's a lot of times when I realize I can't really sit up, then just go home after hours of suffering and possibly infecting my coworkers. (I'm actively trying to be better at this.)
Tonight, the guy said, "I'm super tired. I just want to read stories and go to bed." Sure enough, he had a temperature.