The mumps virus has a special affinity for testicular tissue and can easily reach the testes through the bloodstream, causing testicular inflammation, resulting in mild testicular atrophy and progressive fibrosis, which can reduce fertility and, in severe cases, lead to lifelong infertility. However, not all patients who have had mumps are unable to have children in the future. This is because: 1. The blood-testis barrier is an effective immune barrier that protects the testes from harmful foreign substances and mutagens; in early childhood, when the blood-testis barrier is not yet formed, the mumps virus damages testicular tissue directly, with serious consequences. However, mumps with orchitis is more common in adolescent and adult men and less common in children. This may be related to the fact that the reproductive system is not well developed during childhood. The mumps combined with orchitis is most common in those with unilateral testicular involvement, and the lesions mostly result in reduced sperm counts and compensatory sperm production in the other testicle, which generally does not affect fertility. 3. The spermatogenic capacity of the testes Each gram of testicular tissue can produce and release 10 million sperm a day. Even if the mumps virus damages both testicular tissues, as long as there is a tiny bit of intact testicular spermatogenic tissue, the testes will compensate for spermatogenesis and can meet fertility targets.