Acute tonsillitis itself is usually not serious but can lead to complications if left untreated. Swollen tonsils can block the airway and cause airway obstruction leading to sleep apnoea.
The infection may spread to the surrounding tissue and neck and result in the collection of pus between the tonsil and tissues around it (peritonsillar infection and abscess). It can also spread to the neck causing pus collection in the neck (parapharyngeal abscess and retropharyngeal space abscess). You can present with difficulty swallowing, drooling, stiff neck or pain and difficulty breathing.