This is an infection that occurs inside the eyeball, usually after an eye surgery or penetrating trauma. Internal ocular infection is dangerous as the eye is essentially a “big ball of water” and can quickly turn into an abscess. The vitreous gel does not have a vigorous immune response so that bacteria can replicate at whim and without tissue in the way to slow down growth. You can compare the eye to a swimming pool … when a pool starts to turn green, it can go bad very fast as the algae replicates and spreads within the water. Endophthalmitis infection is rare these days thanks to modern surgical methods and prophylactic antibiotic coverage. If we suspect an infection, however, this usually means a trip to see our retinal specialist colleagues for a tap and inject (remove a sample for culture and injection of antibiotics inside the eye). If enough pus forms inside the eye, a surgical vitrectomy may be required to clean it out.