Final answer:
Chlamydia, caused by the bacterium Chlamydia trachomatis, is the most frequently reported STI in the United States, mostly affecting people under the age of 25.
Step-by-step explanation:
The genital infection that is the most frequently reported infectious disease in the United States, especially prevalent in individuals younger than 25 years, is chlamydia. Caused by the bacterium Chlamydia trachomatis, this sexually transmitted infection (STI) has been reported to have an estimated 2.9 million infections in 2015. Chlamydia is recognized for affecting more young females than males, and it can be transmitted through sexual contact with various genital regions without the necessity of ejaculation. Once treated and cured, individuals can still be reinfected with chlamydia if they come into sexual contact with someone who is infected.