As a personal testimony, about ten months ago user ifktaotc made a very helpful comment regarding testicular cancer and it's prevalance among young men warning that it was likely that at least some of those reading his comment will have it or already do so. https://news.ycombinator.com/item?id=7120102
After reading I checked myself and found what was confirmed a few days later to be a cancerous mass.
I am very fortunate to be alerted to the fact very early as aside from some initial surgery it hasn't affected the rest of my life. Even though the odds of survival overall are high, not knowing it earlier would have required much more intense and painful treatment (chemo, RPLND).