Final answer:
Chlamydia is the fastest-spreading bacterial STI in the United States, caused by the bacterium Chlamydia trachomatis.
Step-by-step explanation:
The fastest-spreading bacterial STI in the United States is Chlamydia. Chlamydia is caused by the bacterium Chlamydia trachomatis and is the most common bacterial STI in the country. Rates of chlamydia are highest in teens and young adults.