I do wish all these studies had other controls - or at least when reported there was some benchmark to other studies.
Is this the only disease to do this? Or does the flu, cold, stomach ache, etc. also lead to a decrease? Is sperm count within a tight band for an average guy throughout a year - or is it constantly rising and falling?
I searched for flu. Couldn’t find much, but generally you would expect a transient effect from fever then a rebound 6-8 weeks out. One well designed study in boars found a total rebound after a decline, and a study in a single human febrile patient found the same.
And no, you wouldn’t expect large fluctuations in healthy people. The longer term reduced outlook seems suggestive of tissue damage.
However, the three month followup was weak. Only 5 out of the 30 subjects. Also, my prior comment was wrong, possible sperm effects can last up to four months after fever: https://academic.oup.com/qjmed/article/114/4/287/6131797
Result? 50% drop in sperm count compared to matched controls!
Recovery to 75% after three months, but much smaller subset. Three months is sufficient time for sperm turnover.