Unfortunately, the virus that causes fever blisters is extremely contagious.  In some cases, you may not even have an active outbreak in order to transmit the virus to someone else.  As you may be aware, cold sores, or fever blisters are caused by the Herpes Simplex virus.  Herpes Simplex is often transmitted by sexual contact, as well as kissing.  Even if you do not have any observable blisters, Herpes Simplex can also be transmitted via contact with towels and other personal hygiene accessories.  It is also important to realize that Herpes Simplex should not be confused with Herpes Zoster, which causes the Chicken Pox and Shingles.

For the most part, you can consider fever blisters to be contagious as long as you can see them, as well as if they still have a crust over them.  Depending on the situation, they may also be contagious while new skin is forming in the afflicted area.  During these times, it will be to everyone's advantage if you have as little physical contact with other people as possible.  This includes refraining from sharing food utensils, as well as anything that may touch you skin or mucus membranes. In fact, if you had cold sores in the past, it will be best for you to refrain from sharing these items as a matter of routine.

Many children are already exposed to Herpes Simplex before they reach the age of ten.  For example, if someone without active cold sores kisses a child, they may still transmit the virus if asymptomatic shedding is occurring.  In a similar way, when children play together, they may touch or rub these blisters, as well as share food and other contaminated items.  While it is not always possible to avoid infecting children, you should still do your best to stay away from them if you know you have active cold sores.

Antiviral medications will kill off some of the virus particles, but not all of them.  Therefore, even if you do take these medications, you will still be contagious if you have visible sores.   Before you use oral antiviral medications, it is also important to realize that they have many side effects.  Depending on the advice you get from your doctor, it may be best to simply avoid taking antiviral therapies, and simply wait for the infection to clear on its own. 

As long as you are in reasonably good health, Herpes Simplex infections will not kill you.  On the other hand, if you are stressed out, or your immune system becomes weak for some other reason, you can rest assured that cold sores will develop.  No matter how you look at it, you will be contagious under these circumstances.  At the very least, if you are aware of your condition, then you should do everything possible to make sure you do not spread it to others.