The great majority of feline respiratory diseases result from two easily transmitted infections: feline viral rhinotracheitis (FVR), caused by a herpesvirus, and feline calicivirus (FCV). FVR and FCV infection result in similar illnesses, characterized by nasal and ocular discharge, conjunctivitis, ulcers of the oral cavity, anorexia, depression and inflammation of the upper respiratory tract. Cats usually recover in 1 to 2 weeks, although cats with FVR can become persistently infected after returning to normal, shedding the virus during periods of stress. FVR can result in abortion of infected fetuses. Kittens are at greatest risk of FVR and FCV because they usually have had no prior vaccination or exposure and are highly susceptible to infection. Chlamydia psittaci bacteria are a less common cause of feline respiratory disease, but can increase the severity of FVR or FCV infection. Vaccines are available for FVR, FCV and Chlamydia psittaci.