Oral herpes (cold sores or fever blisters that are caused by a virus known as herpes simplex) can be transmitted this way. Germs can find a way into a person's bloodstream through a break in the skin or through membranes lining the mouth. Only people who are already infected with a venereal disease can transmit the disease to others. But it's best to play it safe and avoid sharing cosmetics that come in contact with the mouth and lips. Another good precaution? You should replace your toothbrush every three or four months and, for the sake of good hygiene, you should never share your toothbrush with anyone. Keep in mind that many types of STDs are fairly easy to treat, especially in their early stages. If you wonder whether you may have an STD, it's best to see your doctor without delay. He or she can test you for STDs and, if necessary, provide treatment.