Fever blisters are a common name for the characteristic skin lesions that are caused by oral infections of the herpes simplex virus (also known as herpes labialis). Herpes labialis can never be cured completely because the virus is able to hide from the immune system, which means that herpes labialis can recur and come back.
Local Effects
The primary effects of fever blisters are localized to the mouth in the form of the blisters themselves. Fever blisters can form in or around the mouth and may come in the form of a open sore or a small rash. Fever blisters are often filled with a yellow fluid and may crust over, causing a thin yellow veneer to form over them, or they can rupture to reveal pink and healing skin underneath. These blisters can be very painful and itchy and they can also become infected, which can lead to more widespread swelling and pain. After fever blisters disappear they can come back, generally in the same location as before.
Spread
Fever blisters can also be spread easily through skin-on-skin contact or other means. A person with fever blisters can give them to another person if the blistering area touches the other person's mouth, or if they share razors, towels, dishes, toothbrushes, or other items that commonly touch the mouth. In some cases, fever blisters can cause a manifestation of genital herpes since the viruses that cause both are related. Consequently, patients with fever blisters should avoid oral sex during the periods when they have an active outbreak (visible blisters).
Encephalitis
According to the Mayo Clinic, one of the effects of fever blisters is that their underlying cause (the herpes virus) can spread to the brain, causing encephalitis. Encephalitis refers to an inflammation of the brain which is typically a result of a viral infection. Oral herpes (the kind that causes fever blisters) is particularly able to spread to the brain due to its nearby location. Encephalitis can be very mild, but in some cases the disease can be very serious and cause a very high fever, confusion, convulsions, a coma or even death. Encephalitis is most likely to occur in patients with fever blisters when they are under a lot of stress or if they are taking immune-suppressing medications.
