STI Rates in Lake County, Illinois. An explanation of field attributes: Chlamydia- Chlamydia is a common sexually transmitted infection (STI) caused by the bacteria Chlamydia trachomatis. This is a rate per 100,000. Gonorrhea - Gonorrhea is a common sexually transmitted infection (STI) caused by the bacteria Neisseria gonorrhoeae. This is a rate per 100,000.