Chlamydia trachomatis Chlamydia pneumoniae, not C. trachomatis, is associated with adult pneumonia. C. trachomatis can cause pneumonia in infants following acquisition during the passage through an infected bih canal. Streptococcus pneumoniae and Mycoplasma pneumoniae are by far the most common causes of community acquired pneumonia. Haemophilus influenzae is commonly associated with chronic lung disease. Legionella pneumophila is common in ceain geographic areas and has also been associated with outbreaks.