Common Sexually Transmitted Infections

Why This Matters

Sexually transmitted infections (STIs) aren't just a list of diseases to memorize—they represent key concepts you'll be tested on throughout Human Sexuality, including disease transmission mechanisms, public health prevention strategies, the relationship between biology and behavior, and health disparities. Understanding how different pathogens work (bacterial vs. viral vs. parasitic) helps you predict treatment options, recognize why some infections are curable while others require lifelong management, and explain why certain populations face higher risks.

When you encounter STIs on an exam, you're being tested on your ability to connect biological mechanisms to behavioral and social factors. Can you explain why asymptomatic infections pose unique public health challenges? Do you understand why antibiotic resistance matters for bacterial STIs but not viral ones? Don't just memorize symptoms—know what category each infection falls into and what that means for prevention, treatment, and transmission.

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Bacterial Infections: Curable but Increasingly Complicated

Bacterial STIs share a critical feature: they can be cured with antibiotics. However, this doesn't make them simple—rising antibiotic resistance, high rates of asymptomatic infection, and serious complications from untreated cases make these infections major public health concerns.

Chlamydia

• Most commonly reported bacterial STI in the United States—particularly prevalent among sexually active individuals under 25
• Often asymptomatic (especially in women), meaning many cases go undiagnosed until complications like pelvic inflammatory disease (PID) develop
• Curable with antibiotics, but reinfection is common without partner treatment and consistent prevention practices

Gonorrhea

• Caused by Neisseria gonorrhoeae—can infect the genital tract, rectum, and throat depending on sexual practices
• Antibiotic resistance is a growing crisis, with some strains now resistant to multiple drug classes, making treatment increasingly difficult
• Untreated cases increase HIV susceptibility and can cause PID, infertility, and systemic infection if bacteria enter the bloodstream

Syphilis

• Progresses through four distinct stages: primary (chancre sore), secondary (rash), latent (no symptoms), and tertiary (organ damage)
• The "great imitator"—symptoms mimic other conditions, and the latent stage can last years without obvious signs
• Highly treatable with penicillin in early stages, but tertiary syphilis can cause irreversible damage to the heart, brain, and nervous system

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Viral Infections: Manageable but Not Curable

Viral STIs present a fundamentally different challenge: once infected, the virus remains in the body permanently. Treatment focuses on suppressing viral activity and managing symptoms rather than elimination. This distinction is critical for understanding why prevention—including vaccination where available—is so important.

HIV/AIDS

• Attacks CD4 cells in the immune system—without treatment, progresses to AIDS (Acquired Immunodeficiency Syndrome), leaving the body vulnerable to opportunistic infections
• Transmitted through blood, sexual fluids, and vertical transmission (mother to child during pregnancy, birth, or breastfeeding)
• Antiretroviral therapy (ART) can suppress viral load to undetectable levels, preventing progression to AIDS and eliminating transmission risk ("undetectable = untransmittable")

Human Papillomavirus (HPV)

• Over 200 related strains—some cause genital warts, while high-risk strains (especially 16 and 18) can lead to cervical, throat, and anal cancers
• Most sexually active people will contract HPV at some point; the immune system clears most infections within two years without symptoms
• Highly effective vaccines (like Gardasil 9) protect against the most dangerous strains—one of the few STIs with a prevention vaccine available

Herpes Simplex Virus (HSV)

• Two types: HSV-1 (traditionally oral, causing cold sores) and HSV-2 (traditionally genital), though either can infect either location
• Remains dormant in nerve cells for life—outbreaks can recur, often triggered by stress, illness, or immune suppression
• Antiviral medications (like valacyclovir) reduce outbreak frequency and severity and lower transmission risk, but cannot eliminate the virus

Hepatitis B

• Attacks the liver—acute infection can resolve, but chronic infection leads to cirrhosis, liver failure, and liver cancer
• Transmitted through blood and sexual fluids, similar to HIV, making co-infection common in high-risk populations
• Highly effective vaccine available (part of routine childhood immunization)—one of the most successful STI prevention tools in public health

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Parasitic Infections: The Overlooked Category

Parasitic STIs are often forgotten in discussions of sexual health, but they represent an important third category with distinct transmission patterns and treatment approaches.

Trichomoniasis

• Caused by the protozoan parasite Trichomonas vaginalis—the most common curable STI worldwide
• Increases susceptibility to other STIs, including HIV, by causing inflammation that compromises mucosal barriers
• Easily treated with oral antibiotics (metronidazole or tinidazole), but partner treatment is essential to prevent reinfection

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Quick Reference Table

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Self-Check Questions

1. Which two bacterial STIs are most likely to cause pelvic inflammatory disease (PID) if left untreated, and why does their asymptomatic nature make this complication more common?

2. Compare and contrast HIV and Hepatitis B in terms of transmission routes, treatment options, and prevention strategies. Why is Hepatitis B considered a public health success story?

3. A patient tests positive for an STI that progresses through primary, secondary, latent, and tertiary stages. Which infection is this, and why is early detection particularly important?

4. Which STIs have effective vaccines available, and what does this tell you about the difference between viral infections that can versus cannot be prevented through immunization?

5. If an FRQ asks you to explain why some STIs are curable while others require lifelong management, which examples would you use to illustrate the distinction between bacterial and viral infections?