**1. Introduction**

An infected eye is one of the major causes of corneal blindness after cataract in developing countries [1]. This is mainly induced by bacteria, fungi, yeast, and parasites. However, an increasing number of coinfections, which involve the presence of two or more pathogens, in the same place at the same time, affecting one or both eyes, are being reported. Comparing the single-microorganism infections, in which theoretically there is no competition, the coinfec‐ tions have been proven to have more mechanisms competing against the host resources. This competition is the main factor influencing the clinical course and evolution of the infection [2].

Eye coinfections represent a particular challenge for ophthalmologists; coinfections are difficult to diagnose because often the clinical picture is atypical and mimics different clinical pictures. In addition, eye coinfections respond poorly to antibiotics and usually present a slow clinical course. In these circumstances, it is common for patients to receive multiple treatments when they should be receiving a specific treatment; this can cause a refractory infection and need for surgical treatments.

Although eye infections are common in immunocompetent patients, coinfection can compro‐ mise the immune system of the host [3, 4]. Therefore, the strategies used by each microorganism to survive against different treatments and the self-host immune response have important implications for the diagnosis and prognosis of the infection [5].

This chapter provides a systematic review of the frequency and epidemiological characteris‐ tics, with reports of the most common clinical entities, produced by coinfection in the eye.
