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Women whose male partner is uncircumcised have a 5.6-fold increased risk of infection with Chlamydia trachomatis than women whose partner is circumcised . Thus circumcision reduced the risk 82%. This study involved 305 couples in 5 countries from different parts of the world. A species of chlamydia that is not sexually-transmitted (C. pneumoniae) did not differ in frequency between each group, so supporting the biological plausibility of the effect.
Chlamydia trachomatis is the second most frequent STI (HPV being the most common) and is the most common bacterial STI. The World Health Organization estimates there are 92 million new cases annually, with 3 million in the USA , where annual cost for care is $2 billion . Chlamydia trachomatis is responsible for pelvic inflammatory disease that causes infertility, ectopic pregnancy and pelvic pain. It is also a co-factor in HPV-induced cervical cancer and, in both sexes, HIV transmission. In men, just as in women, it can cause infertility, as well as prostatitis and urethral blockage.