Semen - the viral medium

The "blood-testis barrier" actually blocks blood vessels from the tubes where the sperm are made, so that immune cells can't move from the blood into the sperm. Sarah Fecht does a great job of explaining why this system evolved in Popular Science: "White blood cells see the little spermatozoa as invaders — probably because males don’t start producing semen until after the body’s immune system has already been established." 

That's a fascinating piece of information that I had never considered. I also didn't know that the testes were protected from the rest of the body either.