It is puzzling. How can a single cat living inside his owner’s home, full-time, get a cold? We know that 80 to 90% of cat colds are caused by two types of virus: the herpesvirus and the calicivirus. These viruses produce symptoms just like our colds: an acute upper respiratory infection and discharge from eyes nose and mouth.
The disease is transmitted from cat to cat by direct contact with the infected discharge from the eyes, nose and mouth. It is also transmitted by contaminated litter boxes and water bowls. Sometimes a human hand might transmit the disease. Also, rarely, the disease can be transmitted by airborne droplets.
The virus itself can live outside of the cat for up to 10 days but it depends upon the conditions.
The bottom line is that the disease is transmitted from one cat to another and if a cat is living alone inside full-time it is hard to see how the disease can be transmitted to him.
These are some possibilities as I see it. Taking your cat to the veterinary clinic may be a possible source of infection. Even if the cat has not gone to a veterinary clinic the cat’s owner might have gone there and could conceivably pick up the virus on their hands and then transmit it to their cat once they return home. If the cat’s owner has visited a friend and her friend has a cat with cold the virus could be brought back home on the owner’s hands and body. Wash your hands immediately you get home.
If windows are open in the house (protected by mesh to stop the cat getting out) it is conceivable that air droplets can come into the house if a wandering cat outside had a cold.
Visitors to the house may also bring the virus in on their hands if they have a cat who has a cold. People cannot transmit their cold to their cat because the viruses that attack cats do not affect humans and vice versa.
These are some thoughts about how the virus can get to an indoor cat. Indoor cats must get less colds than free roaming outdoor cats. There is much less of a chance for an indoor cat to get a cold or any other contagious disease. This is one big advantage of keeping a cat indoors.