A 17 year study of 25,000 men between 1989 to 2005 showed a precipitous drop in sperm count. Drawing samples from fertility clinics (aren't you glad you aren't that intern?) the percentage of sperm within semen dropped by 1.9% per year and by 33% over that period. The 73.6 million sperm per millilitre of semen in 1989 seems positively luxurious compared to the average of 49.9 million/ml in 2005. Sperm levels have likely fallen more since the study, seeing as the decline was progressive over the 17 years.
This is, in case you were wondering, a serious health issue. Declining sperm rates correlate with an increase in testicular cancer. Sperm rates would have to fall to 33% of 2005 levels to reach infertility levels, but if sperm rates keep falling at the same rates, that will happen in ten years.
Again: it's possible that in ten years, the average male will be medically infertile.
The cause of the disappearing sperm has yet to be determined, but scientists are scrambling to figure it out. For those concerned, general health advice (eat well, get exercise, avoid chemicals) is often given as advice to maintain a healthy sperm count. Also, wear loose underwear.