Cats and ferrets are very particular about their cleanliness. They lick their coat a great deal — to clean it, to rid it of any parasites (sadly often without lasting success) or to remove dead hair.
It is this last point that causes hairballs (trichobezoars): the balls of hair that end up in the animal’s digestive tract, because through all this grooming the animal swallows large quantities of hair.
While the digestive system manages to move most of these foreign bodies through, some of the clumps of hair can lodge — mainly in the stomach — where they can cause a range of problems.
These problems range from the mild to the more serious: bringing food back up shortly after a meal, persistent retching (like having a frog in the throat), then more serious blockage going as far as complete obstruction (more often in older animals).
You can prevent hairballs by brushing the cat or ferret regularly, occasionally giving it a bath if it is moulting heavily (ferrets especially), treating any skin problems that might increase licking, and offering various products or foods that help these hairballs pass through.
There are pastes, given either preventively or as treatment, and tablets too. Some specialist food brands have also developed kibble with a high plant-fibre content that helps the hair work its way out in the stools.
Finally, cat grass — mainly for indoor cats — also helps the cat bring its hairballs back up.
Treatment relies on the same means as prevention, at stronger doses. That said, if the obstruction is severe, surgery to remove the blockage becomes necessary to save the animal (a problem seen more often in older ferrets). This is why prevention, as ever, matters so much.