Tonsillitis is an inflammatory condition of the tonsils due to bacteria, allergies or respiratory problems. When inflamed, tonsils become swollen and red with a grayish or yellowish coating on its surface. Tonsillitis usually begins with a sudden sore throat and painful swallowing. Tonsillitis causes tonsils and throat tissues to swell obstructing air from passing in and out of the respiratory system. The tonsils infection is common in children and teenagers but rare in adults.