On May 18th I was at the peak of health. Prior to my instructional period at the United Airlines Flight Training Center, I went to the company exercise room and pumped out my usual 30 reps of bench press with my bodyweight, then taught ground school and simulator for 8 hours. Like usual, I wore a mask full-time, although the mask requirement had been lifted for several months. In my opinion, the Training Center is a great petri dish to spread Covid and other illnesses, since almost all trainees have traveled by plane within the previous week, and could easily be asymptomatic carriers. Listen to The Covid Flight From Hell for more information on the potential for airline travel exposure. On May 20th I was feeling very tired, and felt like a bad cold was coming on. I was up all night coughing, and at 0230 sent an email to the United Scheduling Department advising them that I would not be able to come to work for my 0700 instructional period. On May 21, I took a Covid test and the results were positive. I immediately quarantined from the rest of my family and contacted my family doctor, who prescribed a 5-day course of Paxlovid. At the end of the five days, I was feeling much better, and tried to resume a normal schedule. I over-did it! My immune resistance was greatly weakened, and the Covid virus that was circulating in my body caused a resurgence of the illness, much stronger this time. And this time I could not take any medication, since Paxlovid is not approved for break-through Covid cases. I have finally tested negative, and am really physically weak. I've learned my lesson, and will not overdo any work until I am fully recovered.