cryptospirosis think thats what its called one knacker stay inside the dog it is a genetic fault , that can and will be passed on if you breed it can be carried by both male and female geneticly , sometimes they drop . , it has been known to couse cancer . some vets say total castration ,, no need if the internal nut is removed it lowers the chances of cancer as tesicles are ment to be outside the body ,and kept cool
This^.