There are many types of Chlamydia, including: Chlamydia trachomatis: the most common type, which is sexually transmitted and causes PID (pelvic inflammatory disease) in women. Chlamydia pneumoniae: a less common type that can cause pneumonia. Chlamydia psittaci: a type that is spread to birds and can cause psittacosis, a serious and sometimes deadly respiratory illness in humans. Chlamydia suis: a type that is spread to pigs and can cause swine flu, a respiratory illness in humans.