I work under the assumption that most people’s observations are incorrect, about themselves and others. Myself included, I have observed a seeming link between improvements in my fitness, progressive increases after weeks or months, but eventually by an infection of some kind, but then it occurred twice in succession then never happened again. That said, let’s examine your hypothesis, for potential causes:
Twenty years ago, when I was working out in my late 20s and early 30s, I would come down with a debilitating period of flu/cold 4 times a winter, and when I moved to working out indoors, and subsequently reduce my workout from 60-90 minutes to <50 that all stopped. Did working out in the cold cause it? Maybe, or did I overtrain, struggling hard to get through the cold air and winter muck?
Exercise itself is unlikely to cause a problem, so more specifically you have:
- Reduced immunity from long workouts, i.e., lasting more than ~40 minutes
- Room ventilation (lack of, or something growing in your system)
- Contact with an infected person of carrier