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Epidemiology
The prevalence of conjunctivitis is related to the underlying cause which can be influenced by the age of the patient as well as the time of year. Acute conjunctivitis is most frequently found in infants, school-age children and the elderly. The most common cause of infectious conjunctivitis is viral conjunctivitis. Conjunctivitis affects many people and imposes economic and social burdens. It is estimated that acute conjunctivitis affects 6 million people annually in the United States. The cost of treating bacterial conjunctivitis alone was estimated to be $377 million to $857 million per year. The most common etiological agent for viral conjunctivitis is human adenovirus (HAdV). , and some of the more common bacteria that can cause conjunctivitis include Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis Contact with contaminated fingers is a common cause of conjunctivitis. Bacteria may also reach the conjunctiva from the edges of the eyelids and the surrounding skin, from the nasopharynx, from infected eye drops or contact lenses, from the genitals or the bloodstream. Infection by human adenovirus accounts for 65% to 90% of cases of viral conjunctivitis. Approximately 1% of all primary care office visits in the United States are related to conjunctivitis. Approximately 70% of all patients with acute conjunctivitis present to primary care and urgent care. The most frequent cause of conjunctivitis is allergic conjunctivitis and it affects 15% to 40% of the population. Allergic conjunctivitis accounts for 15% of eye related primary care consultations - most including seasonal exposures in the spring and summer or perpetual conditions.

One study has found that those who experience seasonal allergic conjunctivitis may be at risk for other conditions to be aware of, including asthma, as well as allergic rhinitis. Another factor that may have an influence on risk for developing allergic conjunctivitis is type 1 diabetes mellitus. 23 cases per 1000 person-years were found in those with type 1 diabetes mellitis, while 13.5 cases per 1000 person-years were found in the controls of this study. This is likely due to the effects that type 1 diabetes mellitus has on the immune system, including the T-helper 1 and 2 cells. Those being exposed to excess levels of certain air pollutants, such as NO2, O3, or increases in temperature have also been shown to increase the number of outpatient visits for allergic conjunctivitis in a study involving over 3 million people, which evaluated the risk involved with many different air pollutants and weather changes. A study in young children in Southwestern China showed that those with seasonal allergic conjunctivitis or persistent allergic conjunctivitis, a longer lasting condition with the same symptoms, also led to a higher incidence of dry eye.

Some seasonal trends have been observed for the occurrence of different forms of conjunctivitis. The occurrence of bacterial conjunctivitis peaks from December to April, viral conjunctivitis peaks in the summer months and allergic conjunctivitis is more prevalent throughout the spring and summer.