Since 1989 sperm count has been decreasing by almost 5% each year. A major factor is endocrine disrupters found in your environment, and that may be difficult to avoid if you don’t live in the middle of nowhere in relative self-sufficiency. Other factors, however, are completely controllable.
Underwear
Wearing (tight) underwear directly affects the health of your sperm. This is what underwear does to your nuts: