Fever blisters, often called cold sores, are fluid-filled blisters that appear on or around the lips. They are a common viral infection caused by the herpes simplex virus type 1 (HSV-1). These blisters are highly contagious. It is a widespread condition, with 50% to 80% of American adults having oral herpes.
Stages of Contagiousness
A fever blister progresses through distinct stages, each with varying levels of contagiousness. The first stage, known as the tingling or prodromal stage, often begins with itching, burning, or tingling around the lips a day or two before any visible signs appear. During this initial phase, the virus has reactivated and is replicating, making transmission possible even before a blister forms.
Following the tingling, small fluid-filled blisters emerge, typically along the lip border. These blisters represent the second stage and are highly contagious due to the presence of the virus within the fluid. Within 24 to 48 hours, these blisters may merge, then rupture, leading to the third stage: oozing and crusting. This “weeping phase” is considered the most contagious period because the open sores leak fluid containing a high concentration of the virus.
After the blisters break, a scab forms over the shallow open sores, which can last for several days. While the risk of transmission significantly decreases once a scab has fully formed, the cold sore can still be contagious. The virus can remain present until the lesion is completely healed, meaning the scab has fallen off and the underlying skin appears normal. The scab can crack or bleed, potentially exposing the virus, especially when eating or smiling.
How Fever Blisters Spread
Fever blisters spread primarily through direct skin-to-skin contact, often through activities like kissing or other intimate physical contact. The virus can also be transmitted by sharing personal items that have come into contact with the blister or saliva. These include eating utensils, drinking straws, cups, towels, and razors. The virus is highly contagious and can enter the body through any small break in the skin, even those not easily noticeable. Although less common than during an active outbreak, HSV-1 can also spread when no visible blister is present, a phenomenon known as asymptomatic shedding.
Preventing Transmission and When Contagion Ends
Preventing the spread of fever blisters involves diligent hygiene and avoiding direct contact during an outbreak. Avoid kissing or any close skin-to-skin contact with others while blisters are present. Sharing personal items like utensils, towels, lip balm, or razors should also be avoided.
Frequent hand washing is important, especially after touching the blister. Hands should be cleaned immediately to prevent spreading the virus to other body parts, like the eyes, or to other people. The contagious period generally lasts until the sore has completely healed, meaning the scab has fallen off and the skin appears normal.