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Fever blisters on the lips can be brought on by a variety of external sources, but the underlying cause is the herpes simplex virus (HSV-1). This virus is most readily spread when fever blisters are present, but may be transmitted even if a person is asymptomatic. While the root cause is contracting HSV-1 from a carrier, external sources like sun damage, stress, or physical trauma can cause the virus to reactivate.
HSV-1 typically goes dormant after the symptoms of the initial infection have subsided. When it reactivates, for whatever reason, it can cause fever blisters on lips. Fever blisters and similar lesions, like canker sores, can also form inside the mouth, on the soft palate, or even in the esophagus. The most common location, though, is the edge of the lips where they meet the skin of the face.
One common cause of fever blisters is damage from the ultraviolet (UV) radiation of the sun. This can be mitigated by wearing sunblock on the lips when outdoors, so that the radiation isn't able to cause the virus to reactivate. Other trauma to the lips can also trigger outbreaks of blisters, so simply avoiding cuts or contusions can help.
Though fever blisters, also known as cold sores, aren't an actual symptom of a cold, the HSV-1 virus is often reactivated by a suppressed immune system. This means that individuals may be more likely to get them during or after a cold, flu, or other illness.
HSV-1 can also be reactivated by a high level of stress, and like fever blisters brought on by illness, this may be difficult to avoid. While there is no cure for the virus, and therefore no cure for fever blisters on the lips, there are ways to manage both the outbreaks and HSV-1 itself. There are vaccines that may make it more difficult to contract HSV-1, and there are antiviral medications available that can help reduce the likelihood of outbreaks.
While they may be similar in appearance, canker sores are a form of mouth ulcer and aren't caused by a virus. The root cause of these sores is unknown, though they can be brought on by many of the same things as fever blisters, such as trauma, stress, or a suppressed immune system. A medical professional can perform tests to determine whether the HSV-1 virus is present, though the testing generally must be done after the outbreak subsides.