Question
What are sexually transmitted diseases (STDs)? Name the major types, their causative agents, symptoms, and prevention strategies.
Solution — Step by Step
Sexually Transmitted Diseases (STDs) — also called Sexually Transmitted Infections (STIs) or Reproductive Tract Infections (RTIs) — are infections primarily transmitted through sexual contact. Some can also be transmitted through contaminated blood, shared needles, or from mother to child during pregnancy or breastfeeding.
In the NCERT Class 12 Biology context, STDs are a key topic under Reproductive Health (Chapter 4) and are directly asked in NEET and CBSE board exams.
| STD | Causative Agent | Type | Key Symptoms |
|---|---|---|---|
| Gonorrhoea | Neisseria gonorrhoeae | Bacteria | Discharge from genitals, burning urination, pelvic pain in women |
| Syphilis | Treponema pallidum | Bacteria | Painless sore (chancre) at infection site; later systemic spread |
| Chlamydia | Chlamydia trachomatis | Bacteria | Often asymptomatic; discharge, pain during urination |
| Genital herpes | Herpes Simplex Virus (HSV-2) | Virus | Painful blisters/sores in genital area; recurrent outbreaks |
| HIV/AIDS | Human Immunodeficiency Virus (HIV) | Virus | Destroys CD4+ T-lymphocytes; leads to immune deficiency (AIDS) |
| Hepatitis B | Hepatitis B Virus (HBV) | Virus | Liver inflammation; jaundice, chronic liver disease |
| HPV/Warts | Human Papillomavirus (HPV) | Virus | Genital warts; certain strains cause cervical cancer |
| Trichomoniasis | Trichomonas vaginalis | Protozoan | Itching, frothy discharge, discomfort |
Note for exams: NCERT specifically mentions gonorrhoea, syphilis, chlamydia, genital herpes, and HIV/AIDS. Know at least these five.
HIV (Human Immunodeficiency Virus) deserves special attention.
HIV infects and destroys CD4+ helper T-lymphocytes (also called T-helper cells). These are essential for coordinating immune responses. As CD4+ count drops:
- Immune system weakens
- AIDS (Acquired Immuno Deficiency Syndrome) develops — defined as CD4+ count below 200 cells/µL (normal: 500–1500 cells/µL)
- Opportunistic infections (tuberculosis, Pneumocystis pneumonia) and certain cancers occur
Transmission routes of HIV:
- Unprotected sex (most common)
- Contaminated blood transfusion or shared needles (important for intravenous drug users)
- Mother to child (vertical transmission): during pregnancy, childbirth, or breastfeeding
Important: HIV is NOT spread through casual contact — hugging, sharing food, mosquito bites, toilet seats.
Personal level:
- Abstinence or mutual monogamy
- Consistent and correct use of condoms (effective against bacterial and viral STDs)
- Avoiding sharing needles
- Testing before unprotected sex with new partners
Medical level:
- Screening blood before transfusion
- Vaccines available for hepatitis B and HPV (Gardasil, Cervarix)
- Antiretroviral therapy (ART) for HIV — suppresses virus, prevents AIDS progression
- Prophylactic antibiotics for bacterial STDs (treated with penicillin or doxycycline)
Social/Awareness level:
- Sex education in schools — NCERT explicitly mentions this as essential
- Removing stigma around STD testing and treatment
- Partner notification and contact tracing
Several STDs are curable if detected early (bacterial STDs like gonorrhoea, syphilis, chlamydia respond to antibiotics). Viral STDs like HIV and herpes have no cure but are manageable with medication.
Danger of delayed treatment:
- Untreated chlamydia/gonorrhoea → pelvic inflammatory disease (PID) → infertility
- Untreated syphilis → cardiovascular and neurological damage
- Untreated HIV → full-blown AIDS with life-threatening complications
This is why the NCERT emphasis on reproductive health awareness is not just academic — early detection and treatment literally saves lives.
Why This Works
STDs can spread silently because many are asymptomatic (showing no symptoms) in early stages, especially chlamydia. This is why awareness and regular screening are emphasised — you cannot rely on symptoms alone to know if you have an STD.
The reproductive health chapter in NCERT makes the biological argument for comprehensive sex education: an informed population seeks testing earlier, uses protection correctly, and reduces transmission rates. Biology and public health are directly connected here.
Alternative Method — Classify by Causative Agent
For NEET MCQs, knowing the causative agent type helps:
- Bacterial STDs: Gonorrhoea, syphilis, chlamydia → treatable with antibiotics
- Viral STDs: HIV, herpes, HPV, hepatitis B → managed but not fully cured
- Protozoan STDs: Trichomoniasis → treated with antiprotozoal drugs (metronidazole)
NEET frequently asks: “Which of the following STDs is caused by a bacterium?” or “Which STD targets CD4+ T-lymphocytes?” For the latter, the answer is HIV. Another common NEET pattern: “Arrange the progression of HIV infection — HIV entry → destruction of T-helper cells → immune deficiency → AIDS.” The timeline is years, not weeks.
Common Mistake
Students often say “AIDS is caused by HIV.” Technically: HIV (the virus) causes infection; AIDS is the disease state that results from advanced HIV infection when the immune system is severely compromised. A person with HIV does not necessarily have AIDS — with proper antiretroviral therapy, many people with HIV never develop AIDS. This distinction matters in NEET MCQs that ask about causative agents.