How do indoor cats get colds?

How do indoor cats get colds? Illustration: Giordano

It is puzzling how indoor cats get colds because colds are transmitted from cat to cat by direct contact with infected discharge from the eyes nose and mouth. The virus can be transmitted through water droplets in the air. My thoughts are that the virus is brought into the home by the cat’s owner on their hands if for example they visit a veterinary clinic or another person who has a cat who has a cold. These are the obvious reasons but an indoor cat is much less likely to get a cold than a outside cat.