History of febrile illness and variation in semen quality.

Plain-language summary

This study looked at what a fever does to semen quality in healthy men. Twenty-seven men with a median age of 24.4 years gave a semen sample every month and kept a daily record of any fever, over 16 months in Copenhagen. What mattered turned out to be not the fever itself but which sperm it caught. A man is making sperm at every stage at once, and each stage takes a known number of days to reach release, so the gap between a fever and a semen sample tells you which stage that sample is reporting on. A sample given about 6 to 30 days after a fever is mostly showing sperm that were in their final shaping stage at the time, and a sample given about 40 days after is mostly showing sperm that were still dividing. A fever during the shaping stage lowered sperm concentration, left fewer normally shaped sperm and left more sperm that could not swim. A fever during the dividing stage lowered concentration too. At other points a fever did not significantly affect any of these. The longer the fever ran, the worse the effect.

Key findings

• A fever during the final shaping stage of sperm production lowered sperm concentration by 35.0%, and a fever during the earlier dividing stage lowered it by 32.6%.
• A fever during the final shaping stage cut the share of normally shaped sperm by 7.4% and raised the share of sperm that could not swim by 20.4%.
• Each extra day of fever cost a further 8.5% of sperm concentration when it landed during the final shaping stage, and a further 7.1% when it landed during the earlier dividing stage.
• A fever at other points during sperm production did not significantly affect any of these measures.
• Men varied widely in how much a fever affected them.