Cold Sore
Not only does a cold sore hurt and look unsightly, it is a clear signal that the immune system is down. 

The Herpes Virus (Type 1) generally hides in tiny nerve fibers (the ones that make you able to feel sensation on your lips) where the immune system’s killer cells cannot reach it to destroy it. When a person gets run down, the herpes virus seizes the chance to make an appearance and cause suffering.  

For many years, the herpes family of viruses has been closely studied and is implicated in illnesses that range from nervous system disorders to cancer, and even heart disease. Most people pick up this form of herpes in childhood. You can transfer the herpes virus from your lip to your eyes, and cause ocular herpes, and even blindness.