Viruses are the smallest of all the infectious agents. They work by invading and taking charge of a cell's genetic and reproductive machinery. They use their newly acquired machinery to reproduce new viruses, which then repeat the process again. Thankfully, this highly effective invasion is stopped by our own immune system, and most viral infections are acute and self-limiting. Occasionally, however, a virus can invade and set up camp in a dormant or latent state. The latent state protects the virus from the immune system. Then, on occasion, the virus "awakens," and an acute viral infection begins. The active virus can be destroyed, but the latent one remains, waiting for another time to reactivate. Recurrences of these viral infections can be common, especially in immunocompromised patients who lack the defenses to fight them off.
There are very few effective antiviral drugs because it is difficult to formulate a drug that eradicates the virus from the cells without also killing the cells themselves. The currently available antiviral therapeutics are mostly effective in treating the herpes virus, of which there are 4 main types. The first is the herpes simplex virus (the cause of cold sores), genital herpes, and herpes simplex keratitis. The second is the varicella zoster virus, the virus causing chicken pox and shingles. The third variety is the Epstein-Barr virus, which causes mononucleosis. Last, is the cytomegalovirus, a common infectious agent in AIDS patients.
There are 3 main antiviral therapeutics for topical ophthalmic use: idoxuridine, vidarabine, and trifluridine. Three others—ganciclovir, foscarnet, and acyclovir—will also be discussed briefly.
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