Cold sores normally first occur on or near your lip. They are caused by a type of herpes virus that is highly contagious and are often caught in childhood from someone who is infected. The virus lies dormant until it is activated, usually at a most inconvenient moment, and starts by tingling or burning near where the blister is going to occur. Within a few hours one or more small blisters form, often swelling the surrounding area, giving rise to a throbbing painful sore.
A mouth ulcer is a painful pale coloured sore on the moist inner skin of the mouth. They make eating and tooth brushing difficult, as food and the brush scrapes against the exposed surface causing pain and discomfort.
Oral thrush is caused by a fungus called Candida that affects moist areas. The mouth is prone to this infection as it is always moist, or should be, in order to chew and start the digestion of food. Candida is a common organism and many people, particularly those who wear dentures, will not show any signs of thrush infection. This is because Candida does not normally overgrow because other organisms such as bacteria prevent it from rapidly growing and causing symptoms.