You are right to be concerned, because constant sneezing in cats can be a sign of a more serious illness and you should take your cat to your vet as soon as possible for a thorough examination. Many sneezing cats have an upper respiratory tract infection caused by a virus (analogous to a common cold in humans) or bacteria and if there are no further symptoms the sneezing should subside within the next 7 to 10 days. However, in cats the two most common causes of sneezing are not the common cold, which might be expected in humans. The rhinotracheitis virus (Herpes-1) and Calicivirus both can also cause conjunctivitis and a fever, even though sneezing may be the only visible symptom. Cats that have the Herpes-1 virus can remain infected for life, with periods of sneezing coming and going (normally in relation to times of stress). Other possible causes include fungal infections, dental disease, nasal polyps, nasal cancer and allergies. Allergies that lead to sneezing in cats are far less common than in humans, although it is still a possible cause. To determine if the cat’s sneezing is due to cancer, tests need to be carried out including an endoscope examination which may require anesthetic.