Sexually Transmitted Infections (STIs): Causes, Transmission and Prevention

What you'll learn

This topic examines the biological basis of sexually transmitted infections, their causative agents, modes of transmission, and prevention strategies. CXC CSEC Biology papers regularly test your ability to identify specific STIs, explain transmission mechanisms, and evaluate prevention methods in both structured and extended response questions.

Key terms and definitions

Sexually Transmitted Infection (STI) — an infection passed from one person to another through sexual contact, caused by bacteria, viruses, or parasites.

Pathogen — a disease-causing microorganism such as bacteria, viruses, fungi or protozoans.

Transmission — the process by which a pathogen is passed from an infected host to a new host.

Incubation period — the time between initial infection and the appearance of symptoms.

Asymptomatic carrier — an infected individual who shows no symptoms but can transmit the pathogen to others.

Condom — a barrier contraceptive device that prevents direct contact between body fluids during sexual activity.

Abstinence — voluntary refraining from sexual activity, providing complete protection against sexually transmitted infections.

Antibiotic resistance — the ability of bacteria to survive and multiply despite exposure to antibiotic drugs that would normally kill them.

Core concepts

Types of STIs and their causative agents

STIs are classified by the type of pathogen responsible:

Bacterial STIs:

• Gonorrhoea — caused by the bacterium Neisseria gonorrhoeae
• Syphilis — caused by the bacterium Treponema pallidum
• Chlamydia — caused by the bacterium Chlamydia trachomatis

Viral STIs:

• HIV/AIDS — caused by the Human Immunodeficiency Virus
• Genital herpes — caused by Herpes Simplex Virus (HSV)
• Genital warts — caused by Human Papillomavirus (HPV)
• Hepatitis B — caused by the Hepatitis B virus

Parasitic/Fungal STIs:

• Trichomoniasis — caused by the protozoan Trichomonas vaginalis
• Candidiasis (thrush) — caused by the fungus Candida albicans (not always sexually transmitted)

CXC examiners expect you to know at least three bacterial and two viral STIs with their causative organisms.

Gonorrhoea — detailed case study

Gonorrhoea remains one of the most common STIs in Caribbean populations, particularly affecting young adults aged 15-24.

Causative agent: Neisseria gonorrhoeae bacteria

Symptoms:

• Males: burning sensation during urination, yellow-white discharge from penis, painful or swollen testicles
• Females: increased vaginal discharge, painful urination, lower abdominal pain, bleeding between periods
• Many females (up to 50%) remain asymptomatic, increasing transmission risk

Transmission routes:

• Unprotected vaginal, anal, or oral sexual contact
• Mother-to-baby during childbirth (causing eye infections in newborns)
• Cannot be transmitted through toilet seats, towels, or swimming pools

Complications if untreated:

• Pelvic inflammatory disease (PID) in females, leading to infertility
• Epididymitis in males, potentially causing sterility
• Increased risk of ectopic pregnancy
• Spread to joints and heart valves in severe cases

Treatment: Antibiotic injections (ceftriaxone) combined with oral antibiotics. Increasing antibiotic resistance has made treatment more challenging in recent years.

HIV/AIDS — detailed case study

Human Immunodeficiency Virus attacks the immune system, specifically CD4 T-lymphocytes (helper T cells), progressively weakening the body's ability to fight infections.

Causative agent: Human Immunodeficiency Virus (retrovirus)

How HIV leads to AIDS:

1. HIV enters the bloodstream and attaches to CD4 T-lymphocytes
2. Virus injects its RNA and reverse transcriptase enzyme into the cell
3. Viral RNA is converted to DNA and integrated into host cell genome
4. Host cell produces new viral particles, which are released
5. Infected T-cells die, progressively reducing immune function
6. When CD4 count drops below 200 cells/mm³, the person has AIDS (Acquired Immunodeficiency Syndrome)

Transmission routes:

• Unprotected sexual intercourse (vaginal, anal, oral)
• Blood-to-blood contact (sharing needles, contaminated blood transfusions)
• Mother-to-child transmission (during pregnancy, childbirth, or breastfeeding)
• Healthcare accidents involving contaminated sharp instruments

HIV is NOT transmitted by:

• Casual contact (hugging, shaking hands)
• Sharing utensils, toilets, or swimming pools
• Mosquito bites
• Saliva, tears, or sweat (viral load too low)

Symptoms progression:

• Initial flu-like illness 2-4 weeks after infection
• Asymptomatic period lasting years (virus multiplying, immune system declining)
• AIDS stage: recurrent infections, rare cancers, severe weight loss, neurological problems

Treatment: Antiretroviral therapy (ART) combining multiple drugs to suppress viral replication. No cure exists, but treatment allows infected individuals to live normal lifespans with undetectable viral loads.

Syphilis progression

Syphilis progresses through distinct stages if untreated:

Primary syphilis (3 weeks post-infection):

• Painless sore (chancre) at infection site
• Chancre heals within 3-6 weeks without treatment

Secondary syphilis (weeks to months later):

• Skin rash, often on palms and soles
• Fever, swollen lymph nodes, sore throat
• Symptoms resolve even without treatment

Latent syphilis:

• No symptoms, but bacteria remain in body
• Can last for years

Tertiary syphilis (10-30 years later):

• Damage to heart, blood vessels, brain, and nervous system
• Paralysis, blindness, dementia, death

Treatment with penicillin antibiotics is highly effective in early stages.

Routes of transmission

Understanding transmission mechanisms is crucial for prevention strategies:

Direct sexual contact:

• Exchange of body fluids (semen, vaginal fluids, blood)
• Contact with infected mucous membranes or skin lesions
• Vaginal, anal, and oral intercourse all carry risk (varying levels)

Vertical transmission:

• Transplacental transmission during pregnancy
• Contact with infected blood during delivery
• Breastfeeding (particularly for HIV)

Blood-borne transmission:

• Sharing needles or syringes among intravenous drug users
• Contaminated blood transfusions (rare in screened blood supplies)
• Needle-stick injuries in healthcare settings
• Sharing razor blades or toothbrushes

Factors increasing transmission risk:

• Multiple sexual partners
• Existing STIs (create breaks in skin/mucous membranes)
• Unprotected sexual intercourse
• High viral or bacterial load in infected person

Prevention strategies

CXC examiners frequently ask candidates to explain and evaluate prevention methods:

Primary prevention (preventing infection):

1. Abstinence — complete avoidance of sexual activity

   • 100% effective against sexually transmitted infections
   • Socially and culturally challenging for many individuals

2. Barrier methods:

   • Male condoms (85-98% effective with correct use)
   • Female condoms (79-95% effective)
   • Dental dams for oral sex
   • Prevent exchange of body fluids and direct skin contact

3. Monogamous relationships:

   • Both partners tested and negative before sexual activity
   • Exclusive sexual relationship maintained
   • Requires trust and communication

4. Vaccination:

   • HPV vaccine (protects against genital warts and cervical cancer)
   • Hepatitis B vaccine
   • Administered to young people before sexual activity begins

5. Screening and testing:

   • Regular STI testing for sexually active individuals
   • Early detection prevents complications and reduces transmission
   • Partner notification and treatment

6. Education programmes:

   • School-based sexual health education
   • Community outreach in Caribbean nations
   • Mass media campaigns promoting safe sex practices

Secondary prevention (managing infections):

1. Early diagnosis and treatment:

   • Prevents progression to complications
   • Reduces infectivity to partners
   • Partner notification and simultaneous treatment

2. Harm reduction:

   • Needle exchange programmes for drug users
   • Safe blood screening procedures
   • Post-exposure prophylaxis (PEP) after potential HIV exposure

3. Treatment adherence:

   • Complete antibiotic courses even if symptoms resolve
   • Regular ART for HIV to achieve undetectable viral load
   • Follow-up testing to confirm cure

Public health measures in the Caribbean:

Jamaica, Trinidad and Tobago, Barbados, and other Caribbean nations have implemented:

• Free condom distribution programmes
• Anonymous HIV testing centres
• School curriculum on reproductive health
• Television and radio campaigns during health awareness weeks
• Training for healthcare workers in non-judgmental STI services

Social and economic impacts

STIs create significant burdens:

Individual impacts:

• Infertility and reproductive complications
• Chronic pain and illness
• Psychological distress and stigma
• Reduced quality of life

Public health impacts:

• Healthcare costs for treatment and management
• Lost productivity due to illness
• Increased burden on hospital services
• Mother-to-child transmission perpetuating infection cycles

Caribbean health ministries prioritize STI prevention due to limited healthcare resources and the disproportionate effect on young, economically active populations.

Worked examples

Example 1: Structured question on gonorrhoea

Question: A 22-year-old woman visits a clinic complaining of painful urination and abnormal vaginal discharge. Tests confirm gonorrhoea infection.

(a) Name the causative organism of gonorrhoea. [1 mark]

(b) State TWO ways this infection could have been transmitted to the patient. [2 marks]

(c) Explain why many females with gonorrhoea do not seek treatment until complications develop. [2 marks]

(d) Describe TWO complications that may arise if this infection remains untreated. [4 marks]

Model answers:

(a) Neisseria gonorrhoeae [1 mark for correct genus and species]

(b)

• Unprotected sexual intercourse with an infected partner [1 mark]
• Unprotected oral or anal sex with an infected partner [1 mark] [Accept any two valid transmission routes]

(c) Many females infected with gonorrhoea are asymptomatic [1 mark], meaning they show no symptoms and do not realize they are infected, so they do not seek medical help [1 mark].

(d)

• Pelvic inflammatory disease (PID) [1 mark] caused when the bacteria spread to the uterus, fallopian tubes and ovaries, causing chronic pain [1 mark]
• Infertility or reduced fertility [1 mark] resulting from scarring and blockage of the fallopian tubes, preventing egg transport and fertilization [1 mark] [Accept also: ectopic pregnancy risk, spread to joints, newborn eye infections]

Example 2: HIV transmission and prevention

Question: HIV/AIDS remains a significant public health concern in Caribbean countries.

(a) Distinguish between HIV and AIDS. [3 marks]

(b) Explain why a person infected with HIV may transmit the virus to others for several years before developing AIDS. [3 marks]

(c) Evaluate the effectiveness of condoms in preventing HIV transmission. [4 marks]

Model answers:

(a) HIV is the virus (Human Immunodeficiency Virus) [1 mark] that attacks and destroys CD4 T-lymphocytes/helper T cells in the immune system [1 mark]. AIDS (Acquired Immunodeficiency Syndrome) is the disease condition that develops when the immune system is severely weakened and CD4 count drops below 200 cells/mm³ [1 mark].

(b) After initial infection, HIV undergoes a long asymptomatic period/latent phase lasting several years [1 mark] during which the person shows no symptoms and may not know they are infected [1 mark]. However, the virus is actively replicating and the person can transmit the virus through sexual contact, blood contact, or vertical transmission [1 mark].

(c) Effective because:

• Condoms prevent direct exchange of body fluids (semen, vaginal fluids) that contain the virus [1 mark]
• When used correctly and consistently, condoms are 95-98% effective at preventing HIV transmission [1 mark]

Limitations:

• Effectiveness depends on correct use; condoms can break or slip if used incorrectly [1 mark]
• User errors (putting on incorrectly, using oil-based lubricants, reusing) reduce effectiveness [1 mark]

[Accept other valid evaluation points about accessibility, cultural barriers, or need for consistent use]

Example 3: Prevention strategies comparison

Question: A health education programme in Trinidad aims to reduce STI rates among teenagers.

Suggest and justify THREE prevention strategies that should be included in this programme. [6 marks]

Model answer:

1. School-based sexual health education [1 mark] Teaching teenagers about STI transmission routes, symptoms, and consequences enables them to make informed decisions about sexual behaviour and recognize when to seek medical help [1 mark].

2. Free condom distribution and training in correct use [1 mark] Providing barrier contraceptives removes cost barriers and teaching proper use maximizes effectiveness, directly preventing fluid exchange during sexual activity [1 mark].

3. Anonymous, youth-friendly STI testing services [1 mark] Reducing stigma and providing confidential testing encourages teenagers to get tested regularly, enabling early detection and treatment before complications develop and reducing onward transmission [1 mark].

[Accept other valid strategies: abstinence promotion, HPV vaccination, peer education programmes, social media campaigns]

Common mistakes and how to avoid them

• Mistake: Stating that HIV/AIDS can be transmitted through casual contact, sharing utensils, or mosquito bites. Correction: HIV requires direct exchange of body fluids (blood, semen, vaginal fluids, breast milk) and cannot survive outside the human body. It is transmitted only through sexual contact, blood-to-blood contact, or mother-to-child transmission.

• Mistake: Confusing HIV with AIDS and using the terms interchangeably. Correction: HIV is the virus that causes infection; AIDS is the syndrome that develops years later when the immune system becomes severely compromised. A person can be HIV-positive for many years without having AIDS.

• Mistake: Claiming that bacterial STIs cannot be cured or that viral STIs can be completely cured with antibiotics. Correction: Bacterial STIs (gonorrhoea, syphilis, chlamydia) can be cured with appropriate antibiotic treatment. Viral STIs (HIV, herpes, HPV) cannot be cured but can be managed; antibiotics do not work against viruses.

• Mistake: Writing that all STI-infected individuals show obvious symptoms. Correction: Many STIs, particularly chlamydia and gonorrhoea in females, are asymptomatic. Infected individuals can transmit the pathogen without knowing they are infected, which is why regular screening is important.

• Mistake: Stating vague prevention methods like "be careful" or "practice safe sex" without explaining specific mechanisms. Correction: Exam answers require precise prevention strategies: use barrier methods (condoms), maintain monogamous relationships with tested partners, complete vaccination courses (HPV, Hepatitis B), practice abstinence, or undergo regular STI screening. Explain how each method prevents transmission.

• Mistake: Describing only immediate symptoms without mentioning long-term complications of untreated STIs. Correction: CXC questions often ask about consequences of untreated infections. Include complications such as infertility (from PID), ectopic pregnancy, neonatal infections, organ damage in tertiary syphilis, or progression to AIDS in HIV infection.

Exam technique for STIs questions

• Command word "Explain": Requires both stating what happens AND providing biological reasoning. For transmission questions, name the route (e.g., "unprotected sexual intercourse") AND explain the mechanism (e.g., "allows infected body fluids containing pathogens to enter mucous membranes"). Each explanation point typically earns 2 marks: 1 for statement, 1 for reasoning.

• Command word "Suggest prevention strategies": Provide specific, practical methods with justification. Generic answers like "be more careful" earn no marks. State the method (abstinence, condom use, vaccination, testing) and explain how it prevents transmission or infection. Questions asking for multiple strategies require distinct methods, not variations of the same approach.

• Distinguish/Compare questions: Structure answers clearly showing the difference between two concepts (HIV vs. AIDS, bacterial vs. viral STIs). Use comparative language: "HIV is... whereas AIDS is..." or create a two-column format. Each distinguishing point earns a mark, typically requiring 2-3 distinct differences.

• Extended response questions (6-8 marks): Organize answers logically with subheadings or numbered points. Include causative organisms, transmission mechanisms, symptoms, complications, and prevention/treatment as appropriate to the question. Use precise biological terminology and Caribbean examples where possible. Allocate approximately 1 minute per mark for planning and writing.

Quick revision summary

STIs are infections transmitted through sexual contact, caused by bacteria (gonorrhoea, syphilis, chlamydia), viruses (HIV, herpes, HPV), or parasites. Transmission occurs through exchange of body fluids or direct contact with infected tissues. HIV destroys T-lymphocytes, progressively weakening immunity until AIDS develops. Many infections are asymptomatic, particularly in females, enabling silent transmission. Prevention includes abstinence, barrier methods (condoms), monogamous relationships, vaccination (HPV, Hepatitis B), and regular screening. Bacterial STIs are curable with antibiotics; viral STIs are manageable but incurable. Untreated infections cause infertility, organ damage, and increased mortality.