Key Facts about Sexually Transmitted Infections

Why This Matters

Understanding sexually transmitted infections (STIs) goes far beyond memorizing a list of diseases—you're being tested on the biological mechanisms of transmission, the public health implications of infection patterns, and the intersection of behavior, biology, and social factors that influence sexual health outcomes. STIs represent a critical case study in how pathogens exploit human behavior, how prevention strategies work at both individual and population levels, and why certain infections persist despite available treatments.

When you encounter exam questions about STIs, you'll need to distinguish between bacterial vs. viral infections (which determines treatment options), understand why some infections are asymptomatic (and how this affects transmission dynamics), and recognize how factors like stigma, access to healthcare, and vaccination shape disease prevalence. Don't just memorize symptoms—know what type of pathogen causes each infection, why that matters for treatment, and what public health strategies target each one.

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Bacterial STIs: Curable but Persistent

Bacterial infections can be eliminated with antibiotics, yet they remain among the most common STIs. The key challenge is that many bacterial STIs are asymptomatic, allowing transmission to continue undetected.

Chlamydia

• Most frequently reported bacterial STI—often called a "silent" infection because up to 70% of cases in women and 50% in men show no symptoms
• Untreated infections cause serious complications including pelvic inflammatory disease (PID), ectopic pregnancy, and infertility in all genders
• Partner notification and simultaneous treatment are essential to prevent reinfection, highlighting the relational nature of STI management

Gonorrhea

• Antibiotic resistance is a growing crisis—dual antibiotic therapy is now standard because single-drug treatments increasingly fail
• Can infect multiple sites including the genital tract, throat, and rectum, meaning transmission can occur through various sexual practices
• Complications mirror chlamydia with PID in women and epididymitis in men, often occurring as co-infections

Syphilis

• Progresses through four distinct stages—primary (chancre sore), secondary (rash), latent (no symptoms), and tertiary (organ damage)
• "The great imitator" because its varied symptoms mimic other diseases, making clinical recognition challenging without testing
• Penicillin remains highly effective even after decades of use, unlike gonorrhea's resistance patterns—a key comparison point for treatment discussions

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Viral STIs: Manageable but Not Curable

Viral infections cannot be eliminated from the body with current medicine. Treatment focuses on suppressing viral activity, managing symptoms, and reducing transmission risk.

HIV/AIDS

• Targets CD4 immune cells specifically—progressive destruction leads to AIDS (Acquired Immunodeficiency Syndrome) when the immune system can no longer fight infections
• Multiple transmission routes include unprotected sex, shared needles, and vertical transmission from mother to child during birth or breastfeeding
• Antiretroviral therapy (ART) transforms prognosis—effective treatment allows normal lifespan and reduces viral load to "undetectable = untransmittable" (U=U) levels

Genital Herpes

• Caused by herpes simplex virus (HSV)—HSV-2 typically causes genital infections, though HSV-1 (usually oral) increasingly causes genital cases
• Lifelong latent infection means the virus remains dormant in nerve cells with potential for recurrent outbreaks triggered by stress, illness, or immune suppression
• Asymptomatic shedding allows transmission even without visible sores, complicating prevention efforts

Human Papillomavirus (HPV)

• Most common STI overall—with over 200 strains, some cause genital warts while high-risk strains (16, 18) are linked to cervical, anal, and oropharyngeal cancers
• Most infections clear spontaneously within two years due to immune response, but persistent high-risk infections can progress to cancer over decades
• Vaccination is highly effective and recommended before sexual debut—a key example of primary prevention in sexual health

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Vaccine-Preventable STIs

Some STIs can be prevented through vaccination, representing a major public health achievement. Vaccines work by training the immune system to recognize and fight specific pathogens before exposure.

Hepatitis B

• Attacks liver cells specifically—chronic infection can lead to cirrhosis, liver failure, and hepatocellular carcinoma (liver cancer)
• Transmitted through infectious body fluids including blood, semen, and vaginal secretions, with higher viral concentration than HIV in blood
• Universal infant vaccination has dramatically reduced new infections—a model for STI prevention through immunization

Human Papillomavirus (HPV)

• Gardasil 9 protects against nine strains—including the two responsible for ~70% of cervical cancers and two causing 90% of genital warts
• Recommended ages 11-12 with catch-up through 26—effectiveness decreases after sexual debut due to likely prior exposure
• Demonstrates primary prevention principle—stopping infection before it occurs rather than treating after transmission

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Parasitic STIs: Often Overlooked

Parasitic infections are caused by organisms that live on or in a host. Unlike bacteria and viruses, parasites are eukaryotic organisms—single-celled protozoa in the case of trichomoniasis.

Trichomoniasis

• Most common curable STI worldwide—caused by the protozoan parasite Trichomonas vaginalis, which infects the urogenital tract
• Highly gendered symptom presentation—women often experience itching, burning, and frothy discharge while men are typically asymptomatic carriers
• Increases susceptibility to other STIs including HIV, demonstrating how one infection can facilitate another—a concept called biological synergy

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Quick Reference Table

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Self-Check Questions

1. Which two bacterial STIs share similar complications (PID, infertility) and often occur as co-infections, yet differ significantly in their response to antibiotic treatment?

2. Compare and contrast HIV and genital herpes: What do they share as viral infections, and how do their mechanisms of harm differ?

3. A public health campaign wants to reduce STI-related cancers. Which two infections should they prioritize for vaccination programs, and why?

4. Why does the asymptomatic nature of chlamydia, gonorrhea, and trichomoniasis pose a greater public health challenge than symptomatic infections like primary syphilis?

5. If an FRQ asked you to explain why some STIs are curable while others are only manageable, which infections would you use as examples for each category, and what biological principle explains the difference?