Flu season is also cold sore season for many celebrities.

Cold sores, also known as fever blisters, are caused by a virus and usually appear around the mouth, inside the nose or on the lips. Cold sores are highly contagious and can spring up at any time, a horrible surprise for many celebrities.

The herpes virus usually first occurs in childhood, but can remain dormant until a trigger causes an outbreak. Here are typical triggers that can cause an eruption on the lips — and the unfortunate celebrities who must grin and bear it for the eagle-eyed paparazzi.

Deputy Editor, Rolling Out