chlamydia transmission, which is often known as the silent disease because it has few symptoms, reduces a man's ability to produce children, they found.

 
Research has found Chlamydia damages sperm
 The disease, which is still on the rise in the UK, is more well known for making women infertile if left untreated.

But now researchers, led by Dr Jose Fernandez from Canalejo University Hospital in La Coruna, Spain, have discovered how chlamydia also affects men.

They looked at the damaged sperm of 143 men from infertile couples and compared it with sperm from 50 fertile men.

The infertile men had chlamydia and another common urinary tract infection called Mycoplasma.

The level of damage - or DNA fragmentation - in the infertile men's sperm was more than three times higher than in healthy men.

The concentration of their sperm, its ability to swim quickly and defects in the shape of it were also poor when compared with the healthy volunteers.

The experts then 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.

The findings were released today at the American Society for Reproductive Medicine conference in Washington DC.

Figures published in July by the Health Protection Agency showed a 4% rise in chlamydia between 2005 and 2006, from 109,418 cases to 113,585.

Experts have been particularly concerned about rates of chlamydia among young people, with the NHS launching a national screening programme.

In 2006/07, 115,073 women under 25 were screened but experts are urging more young men to get tested, with only 31,126 screened during the same period.

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.

"The thing that drives most men to sexual health clinics is symptoms, and chlamydia is often symptom-free.

"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 Fernandez said more research was needed to follow up his study.

And he added: "We've developed a new technique that allows us to look at the extent of DNA fragmentation in sperm cells using a microscope. "The purpose of our work was to analyse if there's an increase in fragmentation of DNA with infection.

"It was found after four months of treatment there was a significant decrease in DNA damage that could improve pregnancy rates in these couples.

"Fertility clinics should check for these infections."