Những rào cản này rất hiệu quả. Ví dụ, chỉ một số ít các vi khuẩn có khả năng gây nhiễm trùng giác mạc miễn là bề mặt ngoài của giác mạc biểu mô vẫn còn nguyên vẹn. Thứ hai, hệ thống miễn dịch của cơ thể được bảo vệ chống lại một loại vi sinh vật cụ thể | 76 Chapter 10 OphA Infection OptT Infection occurs when the body s defenses are overcome and injured by a microorganism. Infections can be caused by bacteria parasites viruses and fungi. Given the number of these microorganisms we are in contact with every day the number of resulting infections is remarkably low. This is because the body has many barriers to infection. These barriers are both nonspecific and specific. Nonspecific defenses include physical barriers provided by skin membranes and secretions such as mucous and stomach acid . These barriers are very effective. For example there are only a handful of bacteria that are capable of causing corneal infection as long as the outer surface of the cornea the epithelium remains intact. Secondly the body s immune system has defenses against a specific microorganism should nonspecific measures fail to contain it. When a microorganism breeches these defenses infection results. Infections can occur in any tissues including those of the eye. In some cases physical barriers may not work. For example if the eye becomes scratched and the epithelium is damaged that person is more likely to develop infection. The same is true after surgery where physical barriers are purposely broken. Certain individuals are more susceptible to getting infections. Diabetics and individuals who are immunocompromised have a poorly functioning immune system and cannot use their specific defenses efficiently. This can be the result of another disease such as the human immunodeficiency virus the virus causing autoimmune deficiency syndrome AIDS . Immunodeficiency can also be caused by medications such as corticosteroids. Whatever the reason when an infection occurs anti-infective therapy should be instituted. Therapy may include antibacterials antivirals antifungals or antiparasitics. For these drugs to be effective they must be able to eliminate the microorganism while causing as little damage as possible to the human host. The ability to .
