The 'carrier state'

Animals that become infected with any of the agents responsible for cat 'flu may continue to be a risk to other cats, whether or not they show signs of the disease

This "carrier state" varies, depending on which infectious agent is responsible, but carrier cats are very common in the population. One UK study, for example, suggested that 26% of cats test positive for calicivirus (Binns et al, 2000); in another, 11% of all cats tested positive for Bordetella (Binns et al, 1999). This carrier state is fundamental to the way in which these infections persist in the population and is why control in the multi cat environment can be problematic.