Chlamydia infection is frequently asymptomatic. Many infected individuals are not aware they are infected, and so would not normally get tested.
Asymptomatic individuals are more likely to transmit the infection to others through unprotected sexual contact.
Many commonly used antibiotics are ineffective against Chlamydia trachomatis, due to the absence of peptidoglycan (so beta-lactams don’t work). Other antibiotics, such as tetracycline and doxycycline are effective but have adverse side effects.