Sexually transmitted infections (STIs) are biological agents—bacteria, viruses, or parasites—passed primarily through sexual contact. These infections are incredibly common, often carrying no noticeable symptoms, which allows them to spread silently. Understanding the true scope of this public health issue in the United States requires considering the total burden of both existing and new infections, moving beyond only reported cases. This comprehensive view clarifies the actual percentage of Americans living with an STI.
Overall Prevalence: Lifetime and Annual Burden
The burden of sexually transmitted infections in the United States is substantial. Public health estimates indicate that approximately one in five people in the U.S. had an STI on any given day in 2018. This represents an estimated 68 million individuals living with an infection, illustrating the overall prevalence of STIs across the country.
Prevalence refers to the total number of existing cases, both old and new, at a specific point in time. Incidence, by contrast, refers only to the number of new infections that occur within a defined period, such as a year.
The Centers for Disease Control and Prevention (CDC) estimates that 26 million new infections were acquired in 2018 alone. This high annual incidence demonstrates rapid, ongoing transmission. The combined impact of these infections places a massive financial strain on the healthcare system.
The total direct lifetime medical costs associated with STIs acquired in that single year are estimated to be nearly $16 billion. This figure does not account for indirect costs, such as lost productivity or non-medical expenses. The volume of existing and new cases underscores that STIs represent a persistent public health challenge.
Statistical Breakdown of Major Infections
The overall prevalence rate is heavily influenced by widespread infections that are often undiagnosed because they are not nationally reported. The human papillomavirus (HPV) is the most common STI, with an estimated 79 million prevalent infections. Genital herpes (primarily HSV-2) is the next largest contributor, accounting for 24 million prevalent cases. These two infections, along with chlamydia and trichomoniasis, make up an estimated 98% of all existing STIs.
While HPV and herpes are the most prevalent, other infections are closely monitored due to their high incidence and potential for severe complications. Chlamydia accounts for the largest proportion of reported STIs, with over 1.6 million cases reported in 2021. This bacterial infection is often asymptomatic and detected through routine screening, particularly among young women.
Reported cases of gonorrhea and primary and secondary syphilis both experienced significant increases in 2020, rising 10% and 7%, respectively. Gonorrhea is the second most common notifiable infection, surpassing 677,000 cases in 2020 and continuing a decade-long upward trend. The surge in syphilis is concerning, leading to a nearly 15% increase in congenital syphilis cases in newborns in 2020.
The human immunodeficiency virus (HIV) represents a smaller but high-impact portion of the STI landscape. Although only 36,136 new cases were reported in 2021, the lifelong nature of the infection makes it the most costly STI. Of the estimated $16 billion in lifetime medical costs for infections acquired in 2018, $13.7 billion was attributed to sexually acquired HIV infections. This concentration of cost illustrates the immense burden of managing a chronic infection.
Tracking and Data Collection Methodology
Public health agencies, including the CDC, rely on different systems to track STIs. For bacterial infections like chlamydia, gonorrhea, and syphilis, a mandatory reporting system is in place. Healthcare providers, laboratories, and state health departments are required to report every confirmed case to the national surveillance system. This mandatory reporting provides a precise count of diagnosed cases for annual surveillance reports.
The overall prevalence figure of one in five Americans is an estimate because many infections are not subject to mandatory reporting. For non-reportable STIs, such as HPV and genital herpes, the CDC uses complex mathematical modeling and data from national health and nutrition surveys. These surveys collect biological samples and health information from a representative sample of the U.S. population.
This dual system—mandatory reporting and estimation—explains why the final overall percentage is an estimate. Many STIs, particularly viral ones, cause no symptoms or are not routinely tested for in clinical settings. The modeling approach accounts for this invisible burden, creating a more accurate picture of the total number of Americans living with an STI.
Disparities in Infection Rates
High rates of STIs are not evenly distributed, but are heavily concentrated in specific demographic groups. Adolescents and young adults are disproportionately affected, with the 15-to-24 age group accounting for nearly half of all new STIs annually. This age group experiences the highest rates due to factors like less frequent screening and biological susceptibility.
Significant disparities are also observed across racial and ethnic lines. Rates of chlamydia, gonorrhea, and syphilis are consistently higher in certain minority groups compared to the White population. These differences reflect long-standing socioeconomic and systemic inequities, not biological factors.
Factors such as limited access to quality healthcare, lack of health insurance, and concentrated poverty directly influence testing availability and treatment adherence. Geographical location also plays a role, with some regions reporting significantly higher combined rates of notifiable STIs than others. These variations underscore how social and economic conditions create barriers that increase health risks for specific populations.