Allergies for cats include all of the same things people might be allergic to - dust, mold, pollen, grass, etc. The primary cause of URI is from viral infection, including feline herpesvirus and feline calicivirus. Neither of these viruses are capable of infecting any species other than cats . When the herpes virus appears at the end of nerve fibers, the signs of the disease flare up. A severe herpes infection can cause the eyes to bulge and in some cases actually rupture.
Cats can also develop herpetic ulcers across the cornea.
Calicivirus is transmitted directly from other infected cats through respiratory secretions and can cause either short-lived or chronic infections. Usually, Mycoplasma and Chlamydia are associated with an abundance of thick green discharge. These bacteria are capable of causing respiratory symptoms all by themselves or in conjunction with viruses and allergies. Bacteria normally found on the skin and in the environment, such as Staph and Strep, cannot induce respiratory symptoms by themselves, but can lead to URI through secondary infection. These bacteria quickly become problematic in a cat whose eyes, nose, or throat is already irritated from virus or allergy.