The common causative organisms of ophthalmia neonatorum include Chlamydia trachomatis (most common), Neisseria gonorrhoeae (most threatening), Haemophilus species, Streptococcus species, Staphylococcus aureus, and viruses such as Herpes simplex (HSV). The "rule of fives" is a mnemonic for predicting the most likely bacterial etiology of the condition. However, there is considerable overlap in times to onset. 0 to 5 days: N gonorrhoeae 5 days to 5 weeks: Chlamydia 5 weeks to 5 years: Streptococcus or Haemophilus influenzae Ref: Levsky M.E., DeFlorio P. (2010). Chapter 2. Ophthalmologic Conditions. In K.J. Knoop, L.B. Stack, A.B. Storrow, R.J. Thurman (Eds), The Atlas of Emergency Medicine, 3e.