Tetanus and Diphtheria are very serious diseases. They are rare in the United States today, but people who do become infected often have severe complications. TD vaccine is used to protect adolescents and adults from both of these diseases. Both diphtheria and tetanus are infections caused by bacteria. Diphtheria spreads from person to person through secretions from coughing or sneezing. Tetanus-causing bacteria enter the body through cuts, scratches, or wounds.