What triggers cold sore (fever blister) breakouts?

Cold sores form when the herpes virus gets reactivated.

After a person's initial herpes simplex virus infection (either subclinical or full-blown) some of the herpes virus particles (virions) will remain in the person's system, and remain there forever.

Most of the time, these virions will lie dormant in the person's system and cause no overt effects. At times, however, they can become active. And when this reactivation occurs the person will experience the outbreak of a cold sore.

What types of events can trigger herpes virus reactivation?

Risk factors for outbreaks.

  • Emotional upset and stress.
  • Physical stress and fatigue.
  • Illnesses (including a cold or the flu).
  • Injury to the lips or skin, such as physical trauma or severe chapping.
  • Injury to the lips from excessive exposure to bright sunlight or ultraviolet lamps.
  • Menstruation or pregnancy.
  • An immune system deficiency.

The timing of fever blister outbreaks is often associated immune system stress.

Monthly web searches for the term 'cold sores' peaks in December.

Several of the factors listed above seem to correlate with time periods when a person's immune system is weakened or stressed.

It's thought that during "normal" times a person's immune system is able to keep the herpes virus in check. However, when it's compromised (like when a person is sick) the herpes virus is able to overcome a person's natural defenses and cause the formation of a lesion.


Take a look at our chart. It shows how frequently people Google for information about cold sores. Notice how this activity peaks in the winter.

You might expect that this time of year is when people become stressed both because of the weather and holidays.

Get the upper hand on managing cold sores by learning when to expect them.

Research has shown that today's antiviral medications for cold sores can significantly minimize outbreak severity and duration if their use is started early enough. This "early" treatment approach is typically also encouraged with OTC products and home remedy treatments.

By observing which factors typically trigger their cold sores a person can learn when to anticipate an outbreak. If a person begins the use of medications early enough (preferably during the Tingle stage) they can quite possibly reduce their cold sore symptoms dramatically.

Where do the dormant herpes virions that cause cold sores reside?

Dormant herpes virus particles reside in the face's nerves.

As we mentioned previously, it's the reactivation of dormant herpes simplex virions that causes the formation of cold sores.

Between outbreak episodes, the dormant virus particles lie quietly "asleep" in nerve tissue. (In the case of HSV1 these dormant virions usually reside in the trigeminal nerve ganglion.) Once reactivated the virus travels down the nerve to the area of the face where the cold sore lesion ultimately forms. (If you're interested, here are some pictures of herpes simplex virions.)

That's why cold sores always reappear in the same places.

In most cases a person's cold sores will always recur in essentially the same general area. That's because that location serviced by the nerve that harbors the dormant herpes virus (see illustration).

How common is it that a person has been exposed to the herpes simplex virus (HSV1)?

It has been estimated that up to 80% of the general population has the presence of herpes simplex virus antibodies in their blood. Antibodies are proof positive that a person has been exposed because our immune system only creates them in response to the physical presence of virions within our bodies.

The good news is, at least from a statistical standpoint, that although about 80% of the general population has been infected with the herpes simplex virus (HSV1) it's in only about one third of these people that the residual virus particles will become active at times and cause recurrent outbreaks of cold sores (fever blisters). The rest of us do still carry the virus but, for whatever reasons, it remains dormant and we remain cold-sore free.

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