STIs — The Definition and the Big List
Infections or diseases that spread through sexual intercourse are collectively called sexually transmitted infections (STIs), also known as venereal diseases (VD) or reproductive tract infections (RTI).
So there are three interchangeable terms for these conditions:
- STI = Sexually Transmitted Infection (the modern term).
- VD = Venereal Disease (the older term).
- RTI = Reproductive Tract Infection (a broader term that also includes infections not transmitted sexually).
The common STIs
The common sexually transmitted infections are gonorrhoea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B and — the most discussed in recent years — HIV, which leads to AIDS.
Here are the 8 common STIs (memorise this list verbatim):
| # | STI | Type of pathogen |
|---|---|---|
| 1 | Gonorrhoea | Bacterium (Neisseria gonorrhoeae) |
| 2 | Syphilis | Bacterium (Treponema pallidum) |
| 3 | Genital herpes | Virus (Herpes simplex virus-2) |
| 4 | Chlamydiasis | Bacterium (Chlamydia trachomatis) |
| 5 | Genital warts | Virus (Human papillomavirus, HPV) |
| 6 | Trichomoniasis | Protozoan (Trichomonas vaginalis) |
| 7 | Hepatitis-B | Virus (HBV) |
| 8 | HIV → AIDS | Virus (HIV) |
HIV is the most discussed STI of recent years. It is covered in detail in Chapter 7 (Human Health and Disease) of Class 12 Biology.
Transmission Routes — Beyond Just Sex
Some of these infections — notably hepatitis-B and HIV — can also be transmitted by sharing injection needles or surgical instruments with infected persons, through transfusion of blood, or from an infected mother to the foetus.
So STIs are transmitted via:
Primary route (all STIs):
- Sexual intercourse (vaginal, oral, anal) with an infected partner.
Secondary routes (specifically for hepatitis-B and HIV):
| Route | Examples |
|---|---|
| Sharing injection needles | IV drug users sharing needles |
| Surgical instruments | Reused unsterilised instruments |
| Blood transfusion | Receiving infected blood (rare now with screening) |
| Mother to foetus | Vertical transmission during pregnancy/birth |
So hepatitis-B and HIV are particularly versatile in transmission. The other STIs are primarily sexually transmitted.
Why does this matter?
- Public health implications: Even people who are not sexually active can get HIV or hepatitis-B (for example, babies born to infected mothers).
- Prevention beyond sex: This calls for needle safety, blood screening, and surgical sterility.
- Universal precautions: Healthcare workers must treat all blood and body fluids as potentially infectious.
Curable vs Incurable STIs
Except for hepatitis-B, genital herpes and HIV infections, the other STIs are completely curable if detected early and treated properly.
So here is the critical distinction:
Incurable STIs (3)
| STI | Type | Why incurable |
|---|---|---|
| Hepatitis-B | Virus | Chronic hepatitis-B persists in the body for life; it can only be suppressed |
| Genital herpes | Virus | HSV-2 establishes a lifelong latent infection in nerve cells |
| HIV / AIDS | Virus | Integrates into host DNA; only suppressible with antiretrovirals |
Their common feature: all three are VIRAL infections that establish chronic or latent infection.
Curable STIs (5)
| STI | Type | Treatment |
|---|---|---|
| Gonorrhoea | Bacterium | Antibiotics (cephalosporins) |
| Syphilis | Bacterium | Penicillin (highly effective) |
| Chlamydiasis | Bacterium | Antibiotics (azithromycin, doxycycline) |
| Genital warts | Virus (HPV) | Topical/surgical removal of warts; HPV itself can clear |
| Trichomoniasis | Protozoan | Metronidazole |
Their common feature: most are bacterial or non-viral (or transient viral), and they respond to antibiotics or other treatment.
Why this distinction matters
- For curable STIs: Early detection plus treatment means a full cure, so getting tested matters.
- For incurable STIs: Prevention is the only effective strategy. Once infected, a person remains a carrier for life.
The takeaway
The three viral STIs that establish chronic infection — hepatitis-B, genital herpes and HIV/AIDS — are the ones we cannot cure; we can only manage them.
Hepatitis-B and HIV/AIDS are covered in more detail elsewhere:
- Hepatitis-B: covered in the Class 12 microbiology and disease chapters.
- HIV/AIDS: covered in detail in Chapter 7 (Human Health and Disease) of Class 12 Biology.
Symptoms, Complications & The Age Group at Risk
The early symptoms of most STIs are minor and include itching, fluid discharge, slight pain and swellings in the genital region. Infected females are often asymptomatic, so the infection may remain undetected for a long time.
Early symptoms (general for most STIs)
- Itching in the genital area.
- Fluid discharge (vaginal or penile).
- Slight pain during urination or intercourse.
- Swellings in the genital region.
A critical asymmetry — females are often asymptomatic
In many STIs, women are more likely to be asymptomatic than men. This causes:
- Delayed diagnosis → complications progress.
- Continued transmission to partners.
- A reservoir of infection in the community.
Complications of untreated STIs
The minor (or absent) symptoms in the early stages, together with the social stigma attached to STIs, deter infected persons from going for timely detection and proper treatment. This can lead to complications later, including pelvic inflammatory disease (PID), abortions, still births, ectopic pregnancies, infertility or even cancer of the reproductive tract.
The six complications:
| Complication | What it is |
|---|---|
| Pelvic Inflammatory Disease (PID) | Spread of infection to the upper reproductive tract (uterus, tubes, ovaries) |
| Abortions | Spontaneous pregnancy loss caused by infection |
| Still births | Foetal death in late pregnancy due to infection |
| Ectopic pregnancies | Embryo implants outside the uterus (tubal pregnancy) — life-threatening |
| Infertility | Permanent inability to conceive (from tubal scarring) |
| Cancer of the reproductive tract | E.g., cervical cancer from HPV |
Each of these is a serious, sometimes life-threatening, condition.
The at-risk age group
Although everyone is vulnerable to these infections, their incidence is reported to be very high among persons in the age group of 15-24 years — the age group to which you also belong.
So the highest-risk age group is 15-24 years — adolescents and young adults.
Why this age group?
- Sexual experimentation without complete information.
- Multiple partners are common.
- Lower use of contraceptives and barriers.
- Reluctance to seek medical care due to stigma.
This is why the chapter directs so much of its awareness messaging at this age group.
Prevention — The 3 Key Principles
There is no reason to panic, because STIs can be prevented. A person can stay free of these infections by following a few simple principles:
(i) Avoid sex with unknown partners or multiple partners.
(ii) Always use condoms during coitus.
(iii) In case of any doubt, go to a qualified doctor for early detection, and get complete treatment if diagnosed with an infection.
The 3 prevention principles
| # | Principle | What it means |
|---|---|---|
| 1 | Avoid sex with unknown/multiple partners | Restrict sexual contact to known, monogamous partners |
| 2 | Always use condoms | A barrier prevents fluid exchange and most skin contact |
| 3 | Seek qualified medical care promptly | Early detection plus complete treatment for curable STIs |
Memorise these three principles verbatim — they're a CBSE favourite.
Why these three?
Principle 1 — Partner selection:
- Most STIs are transmitted via sexual contact.
- Fewer risky partners means less exposure.
- A monogamous, mutual relationship with a tested partner carries the lowest risk.
Principle 2 — Use condoms:
- Condoms physically block fluid and skin contact.
- They are effective against most STIs (especially HIV, gonorrhoea and syphilis).
- They are the only contraceptive that also protects against STIs.
Principle 3 — Early detection and treatment:
- Curable STIs can be cured if detected early.
- Untreated STIs lead to complications (PID, infertility, cancer).
- Even incurable STIs such as HIV and hepatitis-B can be managed if detected early.
Why STIs remain a major problem despite known prevention
- Stigma — people don't talk about STIs openly.
- Asymptomatic infection — many don't know they're infected.
- Risky behaviour — multiple partners and unprotected sex are still common.
- Healthcare access barriers — especially in rural areas.
- Cultural taboos — many communities discourage open discussion of sexual health.
The overall picture
STIs are a major public health threat that:
- Is preventable through informed behaviour.
- Is treatable for most curable STIs.
- Disproportionately affects the 15-24 age group.
- Requires societal awareness and reduced stigma to address effectively.
Small Memory Capsule — Section 8
Lock these in before moving on to infertility and ART.
Three interchangeable terms
STI = VD = RTI.
The 8 common STIs
- Gonorrhoea
- Syphilis
- Genital herpes
- Chlamydiasis
- Genital warts
- Trichomoniasis
- Hepatitis-B
- HIV → AIDS
The most discussed STI
HIV → AIDS — covered in detail in Chapter 7 (Human Health and Disease).
CURABLE vs INCURABLE
| Incurable (3, all viral) | Curable (5, mostly bacterial/protozoan/transient viral) |
|---|---|
| Hepatitis-B | Gonorrhoea |
| Genital herpes | Syphilis |
| HIV / AIDS | Chlamydiasis |
| Genital warts | |
| Trichomoniasis |
The key statement
Except for hepatitis-B, genital herpes and HIV infections, the other STIs are completely curable if detected early and treated properly.
Transmission routes
- All STIs: Sexual intercourse.
- Specifically hepatitis-B and HIV: Also via shared needles, surgical instruments, blood transfusion, and mother-to-foetus.
Early symptoms
- Itching, fluid discharge, slight pain, swellings in the genital region.
- Females are often ASYMPTOMATIC — a major reason for delayed detection.
Complications of untreated STIs
- PID (Pelvic Inflammatory Disease)
- Abortions
- Still births
- Ectopic pregnancies
- Infertility
- Cancer of the reproductive tract
The at-risk age group
15-24 years — adolescents and young adults.
3 prevention principles (drill verbatim)
- Avoid sex with unknown/multiple partners.
- Always use condoms during coitus.
- Consult a qualified doctor for early detection and treatment.
One-line takeaway
STIs (= VD = RTI) include 8 common infections (gonorrhoea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B, HIV/AIDS); 5 are curable but 3 are incurable (hepatitis-B, genital herpes, HIV); untreated STIs cause PID, infertility, ectopic pregnancy and cancer; the 15-24 age group is most at risk; and prevention rests on avoiding unknown partners, using condoms, and seeking timely medical care.
Solved Examples
Example 1: The list of STIs
List the 8 common sexually transmitted infections (STIs). Classify each as curable or incurable.
Answer: The 8 STIs are gonorrhoea (curable), syphilis (curable), genital herpes (incurable), chlamydiasis (curable), genital warts (curable), trichomoniasis (curable), hepatitis-B (incurable) and HIV/AIDS (incurable). The three incurable ones are all viral infections that establish chronic or latent infection, which is why current medicine can only suppress them; the curable five are bacterial, protozoan or transient-viral and respond to antibiotics or removal.
[Board Important] Standard 3-mark CBSE question. Memorise all 8 names plus the curability split.
Example 2: Transmission routes for STIs (beyond sex)
While STIs are primarily transmitted sexually, which STIs can also be transmitted via other routes? List the routes.
Answer: Hepatitis-B and HIV can also spread by routes other than sex: (1) sharing injection needles, (2) shared surgical instruments, (3) transfusion of infected blood, and (4) mother-to-foetus (vertical) transmission. Both persist at high concentration in blood and establish chronic infection, which is why they travel by these blood-borne routes; this versatility makes them harder to control than purely sexually transmitted infections.
[Board Important] Common 2- or 3-mark CBSE question. Always cite both hepatitis-B AND HIV for non-sexual transmission.
Example 3: Curable vs incurable STIs
Distinguish between curable and incurable STIs. Why are some STIs incurable?
Answer: The 5 curable STIs are gonorrhoea, syphilis, chlamydiasis, genital warts and trichomoniasis; the 3 incurable STIs are hepatitis-B, genital herpes and HIV/AIDS. The incurable ones are all viral and establish chronic or latent infection — hepatitis-B persists in liver cells, HSV-2 lies dormant in nerve cells, and HIV integrates its DNA into host CD4 cells — so current drugs can suppress them but not eradicate them.
[Board Important] A favourite CBSE 3-mark question. Always cite the 3 incurable, the 5 curable, and the 'all viral' commonality.
Example 4: Complications of untreated STIs
List the major complications of untreated STIs. Why are these especially dangerous?
Answer: Untreated STIs can lead to (1) PID, (2) abortions, (3) still births, (4) ectopic pregnancies, (5) infertility and (6) cancer of the reproductive tract. These are especially dangerous because they are often irreversible (tubal scarring, lost pregnancies), they fall disproportionately on women — who are more often asymptomatic and so detected late — and they can cause newborn infections and long-term cancers (HPV → cervical cancer, HBV → liver cancer). Early detection and treatment prevent most of them.
[Board Important] Common 3-mark CBSE question. Memorise all 6 complications.
Example 5: The 3 prevention principles + the at-risk age group
State the 3 prevention principles for STIs. Why is the 15-24 age group identified as most at risk?
Answer: The three prevention principles are: (1) avoid sex with unknown or multiple partners; (2) always try to use condoms during coitus; and (3) in case of doubt, consult a qualified doctor for early detection and complete treatment. The 15-24 age group is most at risk because it has the highest rates of sexual experimentation and multiple partners, lower contraceptive and barrier use, limited STI knowledge, and a greater reluctance to seek medical care because of stigma.
[Board Important] Critical CBSE 3-mark question. Memorise all 3 prevention principles verbatim plus the 15-24 age group.