Hypopyon can be present in a corneal ulcer. Behçet's disease, endophthalmitis, panuveitis/panophthalmitis, and adverse reactions to some drugs (such as rifabutin).
Hypopyon is also known as sterile pus, as it occurs due to the release of toxins and not by the actual invasion of pathogens. The toxins secreted by the pathogens mediate the outpouring of leukocytes that settle in the anterior chamber of the eye.