Aphthous Ulcers: These are the most common cause of acute mouth sores, characterized by recurrent, painful ulcers on the mucous membranes of the mouth. They are often triggered by stress, trauma, or certain foods.

Herpes Simplex Virus (HSV): HSV can cause painful vesicles or ulcers in the mouth, especially on the tongue, gums, and the roof of the mouth. It's highly contagious and can be triggered by stress, fever, or sunlight exposure.
Hand, Foot, and Mouth Disease (HFMD): This viral infection is common in children and can cause sores in the mouth, along with a rash on the hands and feet. It's usually caused by coxsackievirus A or enterovirus 71.
Oral Lichen Planus: An autoimmune condition that can cause white, lacy patches; red, inflamed tissues; or open sores in the mouth. It can be painful and may be triggered by certain medications or stress.

Oral Cancer: Although less common, mouth sores that do not heal can be a sign of oral cancer. It's crucial to investigate any persistent sores, especially in individuals with risk factors like smoking or tobacco use.
Infectious Mononucleosis (Mono): Caused by the Epstein-Barr virus, mono can lead to mouth sores, fever, and lymphadenopathy. It's essential to consider this diagnosis, especially in young adults.
Autoimmune Disorders (e.g., Pemphigus, Lupus): Certain autoimmune diseases can cause mouth sores as part of their presentation. These conditions require prompt diagnosis and treatment to prevent serious complications.

Erythema Multiforme: A skin condition that can also affect the mucous membranes, causing mouth sores. It's often triggered by infections or medications.
Behçet's Disease: A rare disorder that causes blood vessel inflammation throughout the body, leading to symptoms like mouth sores, genital sores, and eye inflammation.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Severe skin and mucous membrane disorders usually caused by an adverse reaction to medications or infections. They can cause extensive mouth sores and are medical emergencies.