18 months ago I thought the same and was pretty cynical about the ‘high’ incidence of illness and the need to treat it among the pro’s. It seems mostly allergies (reported as asthma) rather than asthma that afflicts pro cyclists. As a cyclist with a wife in the Pharma business the subject came up from time to time.
Then over the winter of 16/17 I started having sinus issues that wouldn’t go away. Every time I didn’t go to the doc thinking I was about to turn the corner another bout of cold/sinus problems came on. Waited until April before trundling along to the local GP. She diagnosed chronic sinus inflammation and when I mentioned I cycled her response came as a bit of a surprise. As far as she was concerned it was no surprise I was having issues given I cycled a lot, out in all manner of possible allergens (pollen etc) and pathogens (car fumes) for long periods. She prescribed a steroid nasal spray and within two weeks I was back to normal.
It changed my opinion, given the pro’s are out there a good deal longer than I am every week it’s not such a great surprise that many have allergy issues.
How you go about treating those issues, that’s another story.