Sexually transmitted diseases(STDs, venereal diseases) are among the most common infectious diseases in the United States today. STDs are sometimes referred to as sexually transmitted infections, since these conditions involve the transmission of an infectious organism between sex partners. Depending on the disease, the infection can be spread through any type of sexual activity involving the sex organs, the anus, or the mouth; an infection can also be spread through contact with blood during sexual activity. 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.