Hygiene

Cats are known for their fastidious cleanliness. They groom themselves by licking their fur, employing their hooked papillae and saliva. Their saliva is a powerful cleaning agent, but it can provoke allergic reactions in humans. Some people who are allergic to cats—typically manifested by hay fever, asthma or a skin rash —quickly acclimate themselves to a particular animal and live comfortably in the same house with it, while retaining an allergy to cats in general.[46] Many cats also enjoy grooming humans or other cats. Sometimes the act of grooming another cat is initiated as an assertion of superior position in the pecking order of a group (dominance grooming). Some cats occasionally regurgitate hair balls of fur that have collected in their stomachs as a result of their grooming. Longhaired cats are more prone to this than shorthaired cats. Hairballs can be prevented with certain cat foods and remedies that ease elimination of the hair and regular grooming of the coat with a comb or stiff brush. Cats expend nearly as much fluid grooming as they do urinating.

Source : http://en.wikipedia.org/wiki/Cat#Feral_cats