The most common disorder in cats is runny eyes. Infectious organisms such as the flu viruses or chlamydia cause conjunctivitis, an inflammation of the membranes lining the eyelids. This results in a discharge that is either watery and runny or thick and a greenish-yellow color. Occasionally a runny eye is caused by a disorder of the normal anatomy.
Tear glands continually produce secretions that lubricate the surface of the eyeball and flush particulate material into the tear ducts which then drain into the nose. If there's a blockage of the ducts, the tears have no choice but to spill over and run down the face. Blockage can occur if there has been earlier damage to the ducts or if the cat has abnormal anatomy.