There are many different factors that can lower a guy’s sperm count, which in turn can affect his fertility (his ability to get a partner pregnant). These include the use of certain medicines or substances like alcohol, tobacco, marijuana, or other illegal drugs. Sperm count can also be affected by medical conditions like a history of sexually transmitted or other infections, varicocele (swelling of a vein in the testicle), hormone problems, trauma to the testicles, and certain chronic illnesses like celiac disease or cystic fibrosis. Also, the testicles need to be cooler than the rest of the body to produce sperm, so doing things that heat up the testicles like wearing tight underwear (which holds the testicles close to the body) or using hot tubs a lot can decrease sperm count. If you have concerns about your sperm count, it’s a good idea to make an appointment with your health care provider.