Testicular injuries

How really are often situations when someone gets injured, and after that become infertile? What kind of injuries can lead to that stage? Are these injuries usually painful?
I was reading on the internet that there is a possibility (but, highly unusual or unlikely) that significant injury to the testicles can be without any pain? And what could be look like such injury?

[Vana08]

I remember having that conversation with my RE at the fertility clinic..According to what he said,thoses kind of injuries on the man can actually happen sometimes when they are young..It can or not be paintful it depends.. ( If the man has been hurted physically on that area it can be very paintful and damage the canals..)It`s a very sensitive area! Toxic products,smoking and using drugs actually are one of big issues when it comes to infertility..Injuries in man aren`t only physical..The enviroment were he lives in is very important.. Injuries happen most on the side..Well..I dunno ..Hope that helps you to make an idea..!