Testicular immunology
Mumps is a viral disease which causes swelling of the salivary glands and testes. The mumps virus lives in the upper respiratory tract and spreads through direct contact with saliva. Prior to widespread vaccination programs, it was a common childhood disease. Mumps is generally not serious in children, but in adults, where sperm have matured in the testis, it can cause more severe complications, such as infertility.
Sexually transmitted infection
Many STIs are (more easily) transmitted through the mucous membranes of the penis, vulva, rectum, urinary tract and (less often—depending on type of infection) the mouth, throat, respiratory tract and eyes. The visible membrane covering the head of the penis is a mucous membrane, though it produces no mucus (similar to the lips of the mouth). Mucous membranes differ from skin in that they allow certain pathogens into the body. The amount of contact with infective sources which causes infection varies with each pathogen but in all cases, a disease may result from even light contact from fluid carriers like venereal fluids onto a mucous membrane.