When we adopted Archie (4 year old male Siamese) three years ago from the Humane Society, he came to us with an upper respiratory infection and a stubborn eye infection. After treating his eyes with antibiotics and failing to clear up the infection, my husband took his boy to a veterinary ophthalmologist, who said that it was a viral infection rather than a bacterial one. He recommended L-Lysine, which is an over-the-counter vitamine supplement that is supposed to build up the body's defenses against viruses. It worked, and Archie's been free from infection ever since.