In many instances tetanus shots also protect against diphtheria.These are referred to as dT. This helps to maintain diphtheria protection in the general community and as a result cases of diphtheria are virtually unheard of in the US. In a likewise fashion these immunizations are now available as Tdap, adding a pertussis component to the vaccine. The hope is to maintain pertussis(whooping cough) protection levels in the community in general. Protection gained from the childhood series of immunizations had been shown to diminish over time. It can be a serious illness in infants but adults tend to get a prolonged, stubborn cough which can be severe. The translation of the Chinese word for pertussis is "the 100 day cough". Adults over 65 are advised to get one dose of Tdap especially if you have contact with children under 12 months of age. Here is link for more information:
http://www.cdc.gov/vaccines/pubs/vis...is-td-tdap.pdf