Canker sores are painful, round ulcers that form on the soft tissue in the mouth, such as the inside of the cheeks or lips. The ulcers often start with a minor trauma to the mucosal lining ...
The first causes cold sores and fever blisters on the mouth, and is thought to be responsible for most cases of genital herpes in the UK. HSV-2 is most commonly associated with genital herpes alone.
This is because you may have small cuts or sores in your mouth, even if you are not aware of them. There's also risk from a partner's fluids, which may get into contact with your mouth and throat.