feline calicivirus (fcv)

A virus that can cause 'flu like signs such as sneezing, runny eyes and nose and occasionally pneumonia. Often ulceration of the nostrils or inside the mouth occurs. Around half of the cats infected with this virus subsequently become carriers and they shed the virus continuously for long periods of time. There are many different strains of this virus so it can be difficult to control even if cats are vaccinated.

SOURCE: cats.org.uk