BiologyISC Class 12 · Reproductive Health

Sexually Transmitted Diseases (STDs): ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Sexually transmitted diseases spread mainly through sexual contact. Bacterial ones are gonorrhoea, syphilis and chlamydiasis; viral ones are genital herpes, genital warts, hepatitis B and AIDS; trichomoniasis is protozoal. Most are curable if treated early, except hepatitis B, genital herpes and HIV. Condoms and safe practices prevent them.

Sort each STD by its causative agent; red crossed boxes (genital herpes, hepatitis B, AIDS) are not curable, and the viral column holds all three.

Illustration drawn for QuestionBuddy.

Understand Sexually Transmitted Diseases

What STDs are. Infections passed mainly through sexual intercourse are called sexually transmitted diseases (STDs) or infections (STIs), venereal diseases (VD) or reproductive tract infections (RTI). Some, such as hepatitis B and HIV, also spread by sharing injection needles or surgical instruments, by transfusion of infected blood, and from an infected mother to the foetus.

The ISC list, classified by causative agent.

Disease	Type	Causative agent	Typical features
Gonorrhoea	bacterial	Neisseria gonorrhoeae	burning on urination, pus-like discharge
Syphilis	bacterial	Treponema pallidum	painless sore (chancre) on the genitals, later rashes; can spread through the body
Chlamydiasis	bacterial	Chlamydia trachomatis	discharge and pain, often no symptoms in women
Genital herpes	viral	Herpes simplex virus	painful blisters and ulcers that recur
Genital warts	viral	Human papillomavirus (HPV)	small warty growths on the genitals
Hepatitis B	viral	Hepatitis B virus	jaundice, liver damage
AIDS	viral	HIV (a retrovirus)	progressive loss of immunity
Trichomoniasis	protozoal	Trichomonas vaginalis	itching and frothy discharge

Common early symptoms. Itching, fluid discharge, slight pain and swellings in the genital region. Infected women are often asymptomatic, so the disease may stay undetected for long.

Why people delay treatment. Early symptoms are mild or absent, and the social stigma attached to STDs keeps people from seeking timely tests and treatment.

Complications if untreated. Pelvic inflammatory disease (PID), abortions, still births, ectopic pregnancies, infertility, and even cancer of the reproductive tract.

Curability. All of them are completely curable if detected early and treated properly, except hepatitis B, genital herpes and HIV infection. Bacterial STDs respond to antibiotics; the three viral infections named cannot be eliminated by present treatment.

Who is at risk. Everyone can be infected, but incidence is reported to be very high in the 15-24 years age group.

Prevention.
1. Avoid sex with unknown partners or multiple partners.
2. Always use condoms during coitus.
3. In case of doubt, see a qualified doctor early and complete the full treatment.
4. Use sterilised needles and screened blood; avoid sharing needles (hepatitis B, HIV).
5. Vaccination against hepatitis B.

Contraceptives and STDs (frequently tested). Only barrier methods that stop the exchange of body fluids protect: male and female condoms (best protection). Diaphragms, cervical caps and vaults are barriers that cover the cervix, and ISC accepts them as barrier methods that reduce transmission, but they do not cover the whole tract, so condoms are the method to name first. IUDs, oral pills, injectables, implants and surgical methods (vasectomy, tubectomy) give no protection against STDs, because they act on gametes, ovulation or implantation, not on contact between partners.

Place in reproductive health. STDs are a major threat to a healthy society, so prevention and early detection and cure are prime concerns of reproductive health-care (RCH) programmes, alongside sex education in schools.

Flow chart: an untreated STD in a woman. infection → few or no symptoms → no treatment → spread up the tract → pelvic inflammatory disease → blocked or damaged oviducts → ectopic pregnancy or infertility.

Sexually Transmitted Diseases: key facts

STDs by causative agent: $bacterial: gonorrhoea, syphilis, chlamydiasis; viral: herpes, warts, hepatitis B, AIDS; protozoal: trichomoniasis$; Learn the eight ISC diseases in these three groups.
Not completely curable: $hepatitis B + genital herpes + HIV$; All three are viral.
Contraceptives that protect: $condoms (male, female) ≫ diaphragm, cervical cap, vault; IUD, pill, implant, sterilisation: no protection$; Protection comes only from blocking contact with body fluids.
Non-sexual routes: $shared needles + infected blood + mother \to foetus$; Important for hepatitis B and HIV.

How to answer Sexually Transmitted Diseases questions

ISC asks you to classify STDs by causative agent, give symptoms or prevention, explain complications, and pick contraceptives that also prevent STDs.

1Read whether the question wants the disease or the pathogen: 'one bacterial STD' needs a disease name (syphilis), not the organism.
2Classify by agent using the table: three bacterial, four viral, one protozoal.
3For 'which contraceptive also prevents STDs', choose barrier methods (condoms first) and say why: they stop exchange of body fluids.
4For complications, write the chain: untreated → PID → ectopic pregnancy, still birth, infertility, cancer.
5For 'why late detection', give mild or absent symptoms, especially in women, and social stigma.
6For prevention, give the three NCERT principles plus safe needles, screened blood and hepatitis B vaccine.

Sexually Transmitted Diseases in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Choosing contraception that also prevents infection

Suppose a young couple about to marry visit a clinic. One partner had several earlier partners and has not been tested. They want to avoid pregnancy for two years and are considering a Copper-T.

Advise them on contraception and infection risk.

Try it first, then show the solutionSolution

A Copper-T is an effective IUD: copper ions suppress sperm motility and fertilising capacity, and phagocytosis of sperms increases in the uterus.
But an IUD acts inside the uterus on sperms and gives no barrier between partners, so it cannot prevent STDs.
Because one partner may have been exposed, both should first be tested by a qualified doctor and any infection treated completely.
Until tests are clear, they should use condoms, which prevent pregnancy and also block transmission of STDs; later a Copper-T can be used for spacing if both are uninfected.

Answer: Get tested and treated first; use condoms (protect against both pregnancy and STDs) until results are clear; an IUD alone does not prevent STDs.

Memory hook: Only a barrier blocks a bug.

Tricks to remember Sexually Transmitted Diseases

Short memory aids. Say each one aloud once, then use it on a question the same day.

Bacteria = G-S-C
'Good Students Concentrate': Gonorrhoea, Syphilis, Chlamydiasis.
Use it when: Naming bacterial STDs.
Virus = Warts Have Herpes And Hep-B
Warts, HIV/AIDS, Herpes, Hepatitis B: four viral STDs.
Use it when: Naming viral STDs.
The lonely protozoan
Only one protozoal STD: Trichomoniasis, by Trichomonas.
Use it when: Protozoal example.

Sexually Transmitted Diseases: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: One of each kind · 3 marks · Short answerBoard level

Name one bacterial, one viral and one protozoal sexually transmitted disease, with the causative organism of each.

Try it first, then show the solutionSolution

Bacterial: syphilis, Treponema pallidum (or gonorrhoea, Neisseria gonorrhoeae) (1 mark).
Viral: genital herpes, Herpes simplex virus (or genital warts, HPV) (1 mark).
Protozoal: trichomoniasis, Trichomonas vaginalis (1 mark).

Answer: Syphilis (Treponema pallidum); genital herpes (Herpes simplex virus); trichomoniasis (Trichomonas vaginalis).

Example 2: Prevention principles · 3 marks · Short answerBoard level

State three simple principles that can keep a person free of STDs.

Try it first, then show the solutionSolution

Avoid sex with unknown or multiple partners (1 mark).
Always use condoms during coitus (1 mark).
In case of doubt, consult a qualified doctor for early detection and complete treatment (1 mark).

Answer: Avoid unknown/multiple partners; use condoms; seek early diagnosis and full treatment.

Example 3: Contraceptives that also protect · 3 marks · Short answerHarder board level

A couple asks a clinic which contraceptives would also lower their risk of catching an STD. Which two artificial methods would you recommend, how do they give this protection, and why would an IUD or an oral pill not do the same?

Try it first, then show the solutionSolution

Male condom and female condom (diaphragm, cervical cap or vault also accepted as barriers) (1 mark).
They form a physical barrier, so semen and other body fluids carrying pathogens do not pass between partners (1 mark).
IUDs act inside the uterus on sperms and implantation, and pills act on ovulation, implantation and cervical mucus; neither prevents contact between partners' fluids and skin (1 mark).

Answer: Condoms (and other barriers) block fluid exchange; IUDs and pills act on gametes and the uterus, not on contact.

Example 4: Syphilis versus genital herpes · 3 marks · Short answerHarder board level

Compare syphilis and genital herpes under four heads: causative agent, type of pathogen, a typical sign, and whether early treatment can cure the infection completely.

Try it first, then show the solutionSolution

Causative agent and type: syphilis, Treponema pallidum, a bacterium; genital herpes, Herpes simplex virus, a virus (1 mark).
Typical sign: syphilis begins with a painless sore (chancre) on the genitals, followed later by rashes; genital herpes causes painful blisters and ulcers that keep recurring (1 mark).
Cure: syphilis is completely curable if detected early and treated properly (with antibiotics); genital herpes is one of the STDs, with hepatitis B and HIV, that are not completely curable (1 mark).

Answer: Syphilis: bacterium Treponema pallidum, painless chancre, completely curable if treated early. Genital herpes: Herpes simplex virus, recurring painful blisters, not completely curable.

Example 5: Curable or not · 2 marks · Short answerHarder board level

From gonorrhoea, genital herpes, syphilis, hepatitis B, trichomoniasis and HIV infection, pick out those that are not completely curable even with early treatment. What do they have in common, and which of them can also be prevented by vaccination?

Try it first, then show the solutionSolution

Not completely curable: genital herpes, hepatitis B and HIV infection (1 mark).
All three are viral; hepatitis B can be prevented by vaccination (1 mark).

Answer: Genital herpes, hepatitis B, HIV: all viral; hepatitis B vaccine is available.

Example 6: A blood bank case · 3 marks · Short answerHOTS / competency

Suppose a man who has never had sexual contact is found to have hepatitis B, and so is a newborn baby whose mother had the infection. (a) Is hepatitis B then wrongly called an STD? (b) Suggest two ways each infection could have occurred. (c) Give two measures a hospital should take to prevent such cases.

Try it first, then show the solutionSolution

(a) No: hepatitis B spreads mainly by sexual contact, but like HIV it can also spread by other routes, so the term STD describes its main route, not its only route (1 mark).
(b) Man: shared or unsterilised injection needles or surgical instruments, or transfusion of infected blood; baby: from the infected mother to the foetus or at birth (1 mark).
(c) Screen all donated blood; use sterile, disposable needles and instruments; vaccinate health workers and newborns against hepatitis B (any two) (1 mark).

Answer: Hepatitis B is still an STD but also spreads via needles, blood and mother to child; screening, sterile needles and vaccination prevent it.

Where students lose marks in Sexually Transmitted Diseases

The slips examiners see most often, and what to write instead.

Mistake: Naming diseases that are not STDs
In 2025 examiners reported answers such as cholera, pneumonia, common cold, influenza and amoebiasis as examples of STDs.
Do this: Learn the eight ISC STDs in a table by causative agent and use only those.
Usually costs: 1 mark per wrong example.
Mistake: Listing IUDs, pills or sterilisation as preventing STDs
Examiners (2025) found Lippes loop, Cu-T, oral pills and surgical methods given as contraceptives that also prevent STDs.
Wrong working
Cu-T and oral pills prevent pregnancy and STDs.
Right working
Condoms (male or female), and barriers such as the diaphragm, cervical cap and vault.
Do this: Ask: does the method stop body fluids passing between partners?
Usually costs: 1 to 2 marks.
Mistake: Giving the organism when the disease is asked
Examiners (2025) saw causative agents written in place of the STD.
Do this: If asked for a disease, write the disease, and add the organism in brackets if you wish.
Usually costs: ½ to 1 mark.
Mistake: Saying all STDs are incurable
Students generalise from AIDS.
Do this: All are curable if treated early except hepatitis B, genital herpes and HIV.
Usually costs: 1 mark.

Exam tips

Make a three-column table (bacterial, viral, protozoal) and revise it; ISC asks one example of each.
Use the term STD or STI, and mention the alternative names VD and RTI in definition questions.
When asked about prevention, give the three NCERT principles first.
Write 'condoms' first in any list of contraceptives that prevent STDs.
Mention that hepatitis B and HIV also spread through needles, blood and mother to foetus.

Practice questions on Sexually Transmitted Diseases

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which pair consists only of viral sexually transmitted diseases?
(a) Genital herpes and hepatitis B (b) Gonorrhoea and genital warts (c) Chlamydiasis and AIDS (d) Trichomoniasis and syphilis
Hint
Remove every pair containing a bacterium or a protozoan.
Answer
(a) Genital herpes and hepatitis B.
Q2 · 2 marks · Short answerBoard level
List four early symptoms that are common to many STDs.
Hint
All in the genital region.
Answer
Itching, fluid discharge, slight pain and swellings in the genital region.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A woman using an oral contraceptive pill is protected against gonorrhoea.
Reason (R): Oral pills inhibit ovulation and alter cervical mucus.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Does a pill stop bacteria passing between partners?
Answer
(d). R is true, but pills give no barrier, so they do not protect against gonorrhoea.
Q4 · 2 marks · Short answerHarder board level
Name the causative agents of gonorrhoea, genital warts and chlamydiasis, and classify each disease as bacterial or viral.
Hint
Two bacteria, one virus.
Answer
Gonorrhoea: Neisseria gonorrhoeae (bacterial). Genital warts: human papillomavirus, HPV (viral). Chlamydiasis: Chlamydia trachomatis (bacterial).
Q5 · 2 marks · Short answerHarder board level
Why are STD prevention programmes aimed especially at the 15-24 years age group? Give two measures schools can take.
Hint
Incidence; awareness.
Answer
Incidence of STDs is reported to be very high in this age group. Schools can give sex education with correct information on reproductive organs, safe practices and STDs to remove myths, and encourage students to consult a doctor early instead of hiding symptoms.
Q6 · 2 marks · Short answerHarder board level
Explain how an untreated STD can lead to an ectopic pregnancy and to infertility.
Hint
Pelvic inflammatory disease and the oviducts.
Answer
Untreated infection can spread up the female tract and cause pelvic inflammatory disease, which can damage or block the fallopian tubes. A partly blocked tube may hold the embryo so that it implants outside the uterus (ectopic pregnancy); a fully blocked tube stops sperm and ovum from meeting, causing infertility.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a health survey in a district shows that most people with STDs first visit a doctor only after complications such as infertility appear. Suggest three reasons for this pattern and three steps the district health department should take.
Hint
Symptoms, stigma, awareness, facilities.
Answer
Reasons: early symptoms are mild or absent, especially in women; social stigma and fear of disclosure; lack of awareness or of nearby facilities. Steps: awareness campaigns through media and schools; confidential, free testing and treatment centres; free condoms and screening of blood, with sterile needles in clinics (also hepatitis B vaccination).
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a clinic records four patients: P has a painless sore on the genitals; Q has recurring painful blisters; R has frothy discharge and itching, and a microscope shows a flagellated protozoan; S has jaundice after a blood transfusion.
(a) Suggest the likely disease for P, Q, R and S. (2)
(b) Which of these can be completely cured if treated early? (1)
Hint
Chancre; herpes; Trichomonas; liver.
Answer
(a) P syphilis; Q genital herpes; R trichomoniasis; S hepatitis B. (b) P (syphilis) and R (trichomoniasis); genital herpes and hepatitis B are not completely curable.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Name the causative agent of syphilis.Show answer
Treponema pallidum, a bacterium.
2.Which STD is caused by a protozoan?Show answer
Trichomoniasis (Trichomonas vaginalis).
3.Name three STDs that are not completely curable.Show answer
Hepatitis B, genital herpes and HIV infection.
4.Which age group shows a very high incidence of STDs?Show answer
15-24 years.
5.What is PID?Show answer
Pelvic inflammatory disease, a complication of untreated STDs.
6.Which contraceptive gives the best protection against STDs?Show answer
The condom (male or female).

Sexually Transmitted Diseases: frequently asked questions

What are the examples of bacterial, viral and protozoal STDs?

Bacterial: gonorrhoea, syphilis, chlamydiasis. Viral: genital herpes, genital warts, hepatitis B, AIDS. Protozoal: trichomoniasis.

Which STDs cannot be completely cured?

Hepatitis B, genital herpes and HIV infection.

Do IUDs or pills protect against STDs?

No. Only barrier methods, mainly condoms, protect, because they stop exchange of body fluids.

What happens if STDs are not treated?

They can cause pelvic inflammatory disease, abortions, still births, ectopic pregnancy, infertility and even cancer of the reproductive tract.

How can STDs be prevented?

Avoid unknown or multiple partners, always use condoms, consult a doctor early in case of doubt, use sterile needles and screened blood, and get vaccinated against hepatitis B.