What it is
In cats conjunctivitis can be caused by viruses and bacteria, especially the germs behind cat flu, and also by dust, smoke, a foreign body or allergy. Infectious conjunctivitis spreads easily between cats, particularly kittens.
Inflammation in one eye can mean something has got into it, while both eyes inflamed together with sneezing looks more like an infection. In some cats that carry herpesvirus, eye inflammation comes back after stress.
More dangerous eye diseases, such as a corneal ulcer or raised pressure in the eye, can look much the same. So an eye that is very painful, held shut or cloudy needs a vet urgently.
Signs you may notice
The eye waters, sticky or pus-like discharge gathers in its corners, and the eyelids may be stuck together after sleep. The pink tissue around the eye turns red and swollen, and the cat squints and rubs the eye with its paw.
When to see a vet
Book a visit if the eye has watered or produced discharge for more than a day or two. If your cat keeps the eye shut, it looks cloudy or has suddenly changed, contact a vet the same day, because sight can be lost quickly.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.