Cat flu is highly contagious and there are several ways by which it is transmitted from one cat to another. The most common route is by inhaling aerosol droplets that have been sneezed or coughed out by infected cats.
Cats that are ill or suspected to be carriers should be quarantined or isolated, especially if there are other cats in the household.
Maintaining hygiene and sanitation is very important. Food and water bowls, litter trays, toys, and other cat accessories should be washed separately to avoid contamination.
A cat that has been ill with cat flu can become a “carrier cat”; it is when the flu virus remains in the cat’s system, but the cat is not manifesting any symptoms. Shedding of the virus by carrier cats can be intermittent or continuous. But in most cases shedding of the virus occurs during periods of stress. Read more here.