Chlamydia - the most common sexually transmitted infection in the UK - has been shown to damage male, as well as female, fertility.
The infection often goes undiagnosed because there are few obvious symptoms, but is known to cause infertility in women if untreated.
Spanish researchers found it can damage sperm quality.
The study, of 143 men, was presented at an American Society for Reproductive Medicine conference.
"Chlamydia is getting out of control"
A team from Canalejo University Hospital in La Coruna examined sperm from men infected with chlamydia and another urinary tract infection who had failed to father a child.
Using a new microscopic analysis technique, they found the level of damage - or DNA fragmentation - in their sperm was more than three times higher than in healthy men.
The concentration of their sperm, and its ability to swim quickly were also poor, and there were increased levels of defects in shape.
The researchers treated 95 of the infertile men with antibiotics and found their DNA sperm damage improved an average of 36% after four months.
During that period, 13% of the couples got pregnant and, after the treatment was finished, 86% got pregnant.
Growing problem
Rates of chlamydia have risen sharply in the UK in recent years. Research suggests that as many as one in 10 people aged 18 to 25 carry the bacteria which cause the infection.
The NHS has launched a screening programme to try to counter the threat, but there are concerns that too few young people take the problem seriously.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield and Secretary of the British Fertility Society, said more needed to be done to target the younger generation.
He said: "The message is that we might think of chlamydia as a disease that damages female fertility, but we need to think again.
"It does damage female fertility, but it appears to damage male fertility too.
"Chlamydia is getting out of control. We have got to encourage men as well as women to go for screening, but men are more reluctant to do this if they don't have symptoms.
"It is the 18 to 25 age group that is of most concern. There should be a page on Facebook you can log onto and sort screening out."
Dr Pacey said in severe cases chlamydia could cause swelling of the testicles, and the epididymis, the tightly coiled tube which forms a key part of the male reproductive system.
"If you have that you will clearly know about it, you will be complaining quite severely, but in most men it is just a low-level, grumbling disease that does not present them with a great deal of problems.
"That means they do not present for screening, and do not get the treatment that they need."
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