I can concur to your experience. I was knocked out for two weeks-to-month by overdoing strength, interval training and cold showers (~1 year), in combination wi

I can concur to your experience. I was knocked out for two weeks-to-month by overdoing strength, interval training and cold showers (~1 year), in combination with start up life; paradoxically sauna regime helped a lot (rigorous super hot 10-15 minutes + immersed super cold 2 minutes phases). Still, I was catching flu regardless. I also tried 3 days of salt-water fasting to reboot immune system; not sure how much that one did. Cold/coughing after a massive exercise was usually more bronchospasm/exercise induced asthma when I was not used to it. Lack of sleep was always a killer, I had 50% chance I catch some sickness symptoms when sleeping <5 hours within two days, especially when weather was bad.