Original: A sexually transmitted disease ( STD ) , also known as a sexually transmitted infection ( STI ) , or venereal disease ( VD ) , is an illness that has a significant probability of transmission between humans by means of human sexual behavior , including vaginal intercourse , oral sex , and anal sex .
Simplified: A sexually transmitted disease is a disease which generally spreads by sexual intercourse , including oral sex or anal sex . '' Sexually transmitted disease '' is usually written as STD for short .