There are 4 main phases to the life cycle of a cold sore. The herpes simplex virus is contagious during every one of these phases. Together, the 4 phases typically run 7 to 10 days, but can sometimes last up to 2 weeks or longer. It’s important to consult your physician once a cold sore lasts for more than 14 days.
Phase 1 (day 1-2) The tingling phase
Usually, you can feel the skin tightening in the area where the cold sore is about to develop. As a reaction to the infection, the area becomes red and swollen. Even at this early stage, the infection is contagious.
Phase 2 (day 2-4) The blister phase
This is the phase where the cold sore blisters appear. The area turns red and painful. Clusters of small blisters are formed, which can grow into one large blister.
Phase 3 (day 4-5) The ulcer phase
Typically, the most painful, the ulcer stage happens when the blisters burst. This stage is when you develop an open sore and it is the most infectious phase.
Phase 4 (> 5 days) The healing phase
The blisters dry up and a crust forms that is yellow or brown in colour; it forms a scab. As the scab shrinks, you may experience painful cracks that can bleed. A series of scabs will form over the sore, each smaller than the previous one, until the cold sore is completely healed.