Sexually transmitted diseases (STDs, venereal diseases) are among the most common infectious diseases in the United States today.
STDs are infrequently transmitted by other types of contact (blood, body fluids or tissue removed from an STD infected person and placed in contact with an uninfected person).
However, people that share unsterilized needles markedly increase the chance to pass many diseases, including STD's (especially hepatitis B), to others.
Some diseases are not considered to be officially an STD (for example, hepatitis types A, C, E) but are infrequently noted to be transferred during sexual activity.
Consequently, some authors include them as STD's, while others do not.
Some lists of STD's can vary, depending on whether the STD is usually transmitted by sexual contact or only infrequently transmitted.