Herpes Simplex Keratitis: This is often the most common cause of keratitis, especially in patients with a history of cold sores or previous episodes of keratitis. The presentation typically includes a dendritic ulcer on the cornea.

Bacterial Keratitis: Common in contact lens wearers or following trauma, bacterial keratitis can present with significant pain, redness, and discharge. Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus pneumoniae are common pathogens.
Fungal Keratitis: More likely in agricultural workers or after trauma with organic matter, fungal keratitis tends to have a slower onset and may present with feathery or irregular borders of the ulcer.
Acanthamoeba Keratitis: Typically seen in contact lens wearers, especially those who swim or shower while wearing lenses, acanthamoeba keratitis can cause severe pain out of proportion to the clinical appearance.

Syphilis: Although less common, syphilitic keratitis can occur and is crucial to diagnose due to its systemic implications and the need for treatment to prevent long-term complications.
Lyme Disease: Can cause keratitis, especially in endemic areas, and is important to consider due to its potential for systemic involvement and the need for appropriate antibiotic treatment.
Varicella-Zoster Virus (VZV) Keratitis: Similar to herpes simplex, VZV can cause keratitis, particularly in older adults or immunocompromised individuals, and requires antiviral treatment.

Cytomegalovirus (CMV) Keratitis: Primarily seen in immunocompromised patients, such as those with HIV/AIDS or post-transplant patients, CMV keratitis requires specific antiviral therapy.
Mycobacterial Keratitis: Can occur, especially after trauma or surgery, and may require prolonged antibiotic treatment.
Onchocercal Keratitis: Associated with onchocerciasis (river blindness), this is rare outside of endemic areas but can cause significant ocular morbidity if not treated appropriately.