Cold sores are a symptom of the highly contagious herpes simplex virus (which is similar to, but not quite the same as, the STD herpes virus) that affects many people. The virus lives in the body forever, breaking out occasionally into sores in and around the mouth. When you or someone you know has an outbreak, the sores can spread through a number of ways:
The simplest way to spread cold sores is through direct contact. As the sores are in and around your mouth, the virus is spread through kissing or otherwise putting the infected saliva on a person's mouth or skin, particularly skin with an open wound. If you have sores in your mouth, you do not necessarily have the STD herpes (which manifests in outbreaks of sores in the genital area) yourself, but you can give someone else genital herpes through oral sex. Be careful not to put your mouth or the skin with the sore on it anywhere near other people during this time, and likewise avoid the saliva and mouths of infected people when someone else you know has an outbreak.
Saliva can spread onto objects you use on or near your mouth, and anyone else who uses the same objects can come down with the virus. This includes sharing unwashed cups and silverware, as well as lip sticks and balms. You must take extra precaution when you have an outbreak of cold sores not to spread the virus yourself. Following some smart ideas about not sharing items that could result in the spread of the virus (and other contagious viruses) at all times can help prevent you from getting an outbreak if other people are currently contagious.
Touching the Sores and Not Washing Your Hands
Just touching the cold sores can get the virus on your fingers and then whatever you touch afterward can become a source of infection. If another person touches the same item and then touches an open wound or his own mouth, nose, eyes or genital area, the virus can take hold and cause an outbreak. If you have sores, refrain from touching and picking at them, as touching them not only causes the virus to spread, but it also irritates the sores and makes them heal slower (or possibly even with permanent scarring).
Wash your hands frequently if you or someone you know has an outbreak and be careful not to touch wounds or your mouth, nose, eyes or genital area with unwashed hands, as these are common entry points for viruses. The herpes virus that causes sores in your mouth is generally more annoying than harmful, but it can actually have devastating effects in people with lowered immunity, so you must take precautions not to spread the virus by keeping your hands clean and away from sores.
When you have an outbreak of cold sores, you should take precautions to prevent the spread of the virus. These precautions will also help you decrease the risk of irritation and will help your sores heal faster. Just remember that you will likely have to deal with the sores again, as the virus is not curable.