First of all it is important to point out that the herpes virus, and the cold sores that it causes, are often not a sexually transmitted infection. There are actually two different types of the herpes virus. One tends to causes genital sores (these are sexually transmitted usually) and the other tends to cause cold sores (not usually sexually transmitted).
The type of herpes that causes cold sores is very common - in fact, most people are probably infected with it! Unfortunately, the infection can be passed in the saliva, so it is possible to contract the infection, if you have not already been infected, through kissing, sharing tooth brushes and dishes, and other actions that bring you into contact with the saliva of others.
Generally, infection with this kind of herpes is not dangerous and it does not cause more symptoms in otherwise healthy people, other than the occasional cold sore. If you do notice that you are getting a cold sore, you should check in with your primary care doctor
. There are some medications that can be taken to help the sores heal up more quickly; similarly, in people who have frequent flare ups, medications can be taken to reduce the frequency of the flares.