Mumps can certainly cause a low sperm count and, when it does, the effects are usually quite severe. The man can usually remember having painful swollen testicles during his attack of mumps.
No form of treatment will repair damage that has already occurred to the testicles as a result of mumps but even the lowest of sperm counts can now be treated with IVF, with or without ICSI. (ICSI is IntraCytoplasmic Sperm Injection and refers to injecting a single sperm into a single egg.) The results are in the region of one pregnancy in every three treatment cycles in the best centres.