Sexually transmitted infections
An infection occurs when a pathogen enters the body and multiplies, causing harm. A Sexually Transmitted Infection (STI) is specifically defined by its mode of transmission: it spreads primarily through sexual contact.
This contact involves the exchange of infectious fluids or direct skin-to-skin contact with infected areas. The key pathogens involved can be:
- Viruses: e.g., Human Immunodeficiency Virus (HIV), Human Papillomavirus (HPV).
- Bacteria: e.g., Treponema pallidum (causes syphilis), Neisseria gonorrhoeae (causes gonorrhoea).
Why this matters: Understanding that STIs are defined by their transmission route helps distinguish them from other infections spread by air, water, or food. Building on the concept of pathogens, STIs require a specific vector (sexual activity) to move between hosts.
Learning Objective 2: State that HIV is a pathogen that causes an STI.
HIV stands for Human Immunodeficiency Virus. It is a virus (a non-living infectious agent consisting of genetic material inside a protein coat) that specifically targets the human immune system.
- Pathogen: A disease-causing microorganism. HIV is classified as a pathogen because it invades host cells and causes damage.
- STI Status: Because HIV is transmitted through sexual fluids (semen, vaginal fluid, blood), it is classified as an STI.
Learning Objective 3: State that HIV infection may lead to AIDS.
If left untreated, chronic HIV infection progresses to AIDS (Acquired Immunodeficiency Syndrome).
- Mechanism: HIV destroys helper T-cells (a type of white blood cell). As the number of these cells drops, the immune system becomes too weak to fight off other infections.
- Result: The individual develops AIDS, where they become susceptible to opportunistic infections that a healthy immune system would normally prevent.
Learning Objective 4: Describe the methods of transmission of HIV.
HIV is present in specific body fluids: blood, semen, vaginal fluid, and breast milk. For transmission to occur, these infected fluids must enter the bloodstream or mucous membranes of another person.
The primary methods are:
- Unprotected Sexual Contact: Vaginal, anal, or oral sex without a condom allows exchange of semen/vaginal fluids containing HIV.
- Blood-to-Blood Contact: Sharing contaminated needles (e.g., drug injection) or receiving infected blood transfusions.
- Mother-to-Child (Vertical Transmission): During pregnancy, childbirth, or breastfeeding.
| Mechanism |
|---|
| Exchange of semen or vaginal fluid containing HIV enters the partner's body. |
| Direct entry of infected blood into the bloodstream via a contaminated needle. |
| Infected mother passes HIV to infant through breast milk. |
| Receiving blood from an infected donor (rare in screened systems). |
Correction: HIV is not transmitted by insect vectors (like mosquitoes) or casual contact (hugging, shaking hands, sharing toilets). The virus cannot survive long outside the human body and requires direct fluid exchange to infect a new host.
Error: "Using antibiotics cures HIV."
Correction: Antibiotics kill bacteria. HIV is a virus, so antibiotics are ineffective. Antiretroviral therapy (ART) is used to manage HIV, but there is currently no cure.
Learning Objective 5: Explain how the spread of STIs is controlled.
To control an STI, we must break the chain of transmission. This requires explaining how specific actions reduce the probability of pathogen transfer.
Barrier Methods (Condoms/Femidoms):
- Explanation: Latex condoms physically block the exchange of semen and vaginal fluids. Since HIV is carried in these fluids, the barrier prevents the virus from entering the partner's bloodstream or mucous membranes during sex.
- Result: Drastically reduces transmission probability.
Abstinence:
- Explanation: Abstinence means refraining from sexual activity entirely. By removing the behavior that facilitates fluid exchange, the primary route of transmission is eliminated.
- Result: Zero risk of sexual transmission.
Testing and Treatment (Screening):
- Explanation: Regular testing identifies infected individuals early. For bacterial STIs (like chlamydia), antibiotics cure the infection, removing the pathogen from the population. For HIV, antiviral medication reduces the viral load in the blood to undetectable levels, making sexual transmission extremely unlikely.
- Result: Reduces the number of infectious individuals available to spread the disease.
Education and Awareness:
- Explanation: Public education campaigns inform people about how STIs are transmitted and the risks of unprotected sex. Knowledge leads to behavioral changes, such as choosing to use condoms or limiting sexual partners.
- Result: Increases adoption of protective behaviors across the population.
Sterile Needles (for IV drug users):
- Explanation: Providing clean, disposable needles ensures that blood is not shared between users. Since HIV can survive in residual blood on a needle, using a sterile needle prevents direct blood-to-blood contact.
- Result: Prevents non-sexual transmission routes.
Why examiners accept this: Examiners look for the mechanism of action, not just the name of the method. Simply saying 'use condoms' is insufficient for an 'explain' question. You must link the physical barrier to the prevention of fluid exchange.
Correct Usage Example:
- Weak Answer: "Condoms stop HIV."
- Strong Answer: "Condoms act as a physical barrier that prevents the exchange of semen and vaginal fluids between partners, thereby stopping the HIV virus from entering the bloodstream."
Key Phrase to Include: Always mention 'barrier' or 'prevents exchange of fluids' when discussing condoms.
- Use of condoms (barrier method) to prevent exchange of infectious fluids.
- Abstinence from sexual contact to eliminate transmission routes.