Important Diseases Explained
This section deals with some of the most talked-about diseases — diseases that have shaped history, triggered global emergencies, and very regularly featured in UPSC prelims and mains.
Before we dive in, remember one golden rule: every disease has a WHO, WHAT, and HOW — Who causes it (pathogen), What does it do (symptoms/complications), and How does it spread (transmission). If you master this framework for each disease, no question can catch you off guard.
In this section, we will cover some important diseases ranging from viral and bacterial infections to metabolic disorders.
1. Polio (Poliomyelitis)
Polio is one of those diseases that India fought and won — a fact worth celebrating and worth knowing for. Let us understand it.
The Pathogen
Polio is caused by the poliovirus, an RNA virus belonging to the Picornaviridae family.
The name ‘Picorna’ comes from ‘pico’ (very small) + ‘RNA’, so it simply means a tiny RNA virus. It attacks the nervous system and is notorious for causing irreversible paralysis, particularly of the legs.
Transmission Routes
- Faecal-oral route: This is the primary mode — contaminated food, water, hands, surfaces, and objects carry the virus. This is why polio is strongly associated with poor sanitation.
- Respiratory droplets: Droplets from a sneeze or cough of an infected person — less common, but possible.
Key Insight: Polio thrives where sanitation is poor. This is why the Pulse Polio campaign in India was focused on rural and urban slum areas.
Risk Group & Symptoms
Though polio primarily affects children under 5 years of age, any unvaccinated person is at risk. In mild cases, it may be completely asymptomatic or present like a simple flu. In severe cases, however, it can cause permanent paralysis, respiratory failure, and physical deformities — a devastating outcome.
Types of Polio
| Type | What Happens | Key Feature |
| Abortive Poliomyelitis | Flu-like and intestinal symptoms | Short-lived, no lasting effects |
| Non-paralytic Poliomyelitis | Aseptic meningitis (brain meningeal inflammation) | No paralysis |
| Paralytic Poliomyelitis | Virus attacks brain & spinal cord | Paralysis of breathing/speaking/movement muscles |
| Polioencephalitis | Brain swelling | Primarily affects infants |
| Post-Polio Syndrome | Symptoms return years after infection | Muscle weakness and atrophy |
Vaccines
- Inactivated Poliovirus Vaccine (IPV): Uses killed poliovirus. Administered intramuscularly or intradermally (by injection).
- Oral Poliovirus Vaccine (OPV): Uses live, attenuated (weakened) virus. Administered orally — the drops we give to children during Pulse Polio campaigns.
Variants of Poliovirus
| Variant | Full Name | Current Status |
| WPV1 | Wild Poliovirus Type 1 | ENDEMIC — only in Afghanistan & Pakistan |
| WPV2 | Wild Poliovirus Type 2 | ERADICATED |
| WPV3 | Wild Poliovirus Type 3 | ERADICATED |
Fact: India was declared Polio-Free in 2014 by WHO. The Pulse Polio Immunisation Programme was launched in 1995. The Global Polio Eradication Initiative (GPEI) was launched in 1988 by WHO, UNICEF, Rotary International, and the US CDC.
2. Acquired Immunodeficiency Syndrome (AIDS)
AIDS is perhaps the most famous immune-deficiency disease in the world. The name itself tells you the story — Acquired (not inherited), Immunodeficiency (the immune system breaks down), Syndrome (a collection of symptoms). It is caused by HIV, the Human Immunodeficiency Virus — a retrovirus.
What is a Retrovirus? A retrovirus carries RNA as its genetic material. Unlike our cells (DNA → RNA → Protein), a retrovirus reverses this flow: RNA → DNA, using an enzyme called reverse transcriptase. It then inserts this DNA into our cell’s genome and hijacks it.
How HIV Destroys Immunity
HIV specifically targets and destroys T-cells (also called CD4+ cells or Helper T-cells) — the white blood cells that coordinate the entire immune response. As T-cell count falls, the body gradually becomes incapable of fighting even minor infections. When the immune system is severely compromised, the condition is diagnosed as AIDS.
Transmission
- Unprotected sexual contact — the most common mode worldwide.
- Blood-to-blood contact — contaminated needles, syringes, medical instruments, infected blood transfusions.
- Mother-to-child — during pregnancy, childbirth, or breastfeeding.
Important Clarification: HIV is NOT transmitted through casual contact, hugging, kissing, sharing food utensils, or insect bites. This is a common misconception that we should know
Stages of HIV Infection
| Stage | What Happens | Key Point |
| Stage 1: Acute HIV | Flu-like symptoms 2–4 weeks after infection; virus multiplies rapidly | Highly transmissible; lasts few weeks |
| Stage 2: Chronic HIV | Virus active but slow reproduction; may be asymptomatic | Can last 10+ years without treatment |
| Stage 3: AIDS | Immune system severely damaged; susceptible to opportunistic infections & cancers | Survival ~1–3 years without treatment |
Diagnosis Methods
- Antibody Tests: ELISA (Enzyme-Linked Immunosorbent Assay) and Rapid HIV tests — detect HIV antibodies in blood, urine, or saliva.
- Antigen/Antibody Tests: Detect both HIV antibodies AND HIV antigens in blood.
- Nucleic Acid Tests (NATs) / Viral Load Tests: Directly detect HIV RNA in blood — most accurate, tells the quantity of virus.
Treatment
HIV is incurable, but it is manageable. Antiretroviral Therapy (ART) stops HIV from replicating, allowing the immune system to recover and function normally. ART does not eliminate the virus — it suppresses it. Patients on ART can live near-normal lives.
3. Japanese Encephalitis
The word ‘encephalitis’ means inflammation of the brain — and that is exactly what this disease does. It is a severe viral zoonotic disease (zoonotic = passes from animals to humans).
| Aspect | Details |
| Causative Agent | Japanese Encephalitis Virus (JEV) — a Flavivirus (vector-borne RNA virus) |
| Related Viruses | Related to Dengue, Yellow Fever, and West Nile Virus |
| Transmission | Culex species mosquitoes transmit JEV to humans |
| Natural Reservoir | Pigs and aquatic birds are natural reservoirs |
| Symptoms | Mild: fever, headache. Severe: high fever, neck stiffness, disorientation, coma, seizures, spastic paralysis |
| Treatment | No antiviral treatment; only supportive care |
| Prevention | Vaccination: inactivated, live attenuated, and live recombinant vaccine types available |
Vaccine Types — Know the Difference!
Live Recombinant Vaccine: Uses a live, weakened virus/bacteria genetically modified to express an antigen from a different pathogen.
Live Attenuated Vaccine: Uses a weakened form of the actual pathogen itself.
One important point: even survivors of Japanese Encephalitis may be left with permanent cognitive, behavioural, or neurological sequelae — problems like seizures, speech impairment, memory loss, or limb weakness. This is why prevention through vaccination is far more important than treatment.
4. Ebola Virus Disease (EVD)
Ebola is one of the deadliest diseases known to humanity. Its very name evokes fear — and rightly so. It was formerly called Ebola Hemorrhagic Fever because of the severe internal and external bleeding it causes.
| Aspect | Details |
| Causative Agent | Ebola virus — Filoviridae family |
| Natural Host | Fruit bats of the Pteropodidae family |
| Geography | Primarily sub-Saharan Africa; first appeared in South Sudan and DRC in 1976 |
| Transmission | Human-to-human: direct contact with blood/bodily fluids; Animal-to-human: bats, monkeys, apes (hunting/butchering bushmeat) |
| Symptoms | Fever, severe headache, muscle pain, abdominal pain, diarrhoea, vomiting; severe cases: bleeding, organ failure, death |
| Treatment | Two monoclonal antibody treatments: Inmazeb and Ebanga; supportive care (rehydration) |
| Prevention | Two licensed Ebola vaccines currently available |
What are Monoclonal Antibodies?
Monoclonal antibodies (mAbs) are lab-produced proteins that mimic natural antibodies. They target specific antigens to stop a virus from replicating. ‘Monoclonal’ = clones of a single antibody — all identical.
5. Hepatitis
Hepatitis literally means inflammation of the liver (hepar = liver, itis = inflammation). It is an umbrella term — hepatitis can be caused by viruses, toxins, alcohol, or autoimmune conditions. If untreated, it can progress to liver fibrosis, cirrhosis, or liver cancer.
Symptoms include upper abdominal pain, nausea, loss of appetite, fatigue, fever, itchy skin, jaundice (yellowing of skin and eyes), dark urine, and light-coloured stool.
Viral Hepatitis — The Five Types
| Type | Transmission | Acute/ Chronic | Antiviral Tx | Vaccine | Key Fact |
| Hep A | Faecal-oral (contaminated food/water) | Acute only | No | Yes | Occasionally severe |
| Hep B | Blood, sexual contact, mother-to-child | Acute & Chronic | Yes | Yes | Small % → chronic |
| Hep C | Blood (needles, transfusions) | Acute & Chronic | Yes | No | Majority → chronic |
| Hep D | Only in Hep B infected people | Chronic | Yes | No | Severe co-infection |
| Hep E | Faecal-oral (contaminated water/food) | Acute only | No | Some areas | Most prevalent in Asia |
Critical Fact: Hepatitis B and C together account for 96% of overall hepatitis mortality. Hepatitis B is transmitted like HIV.
Hepatitis B has a vaccine; Hepatitis C does NOT.
Non-Viral Hepatitis
- Alcohol-induced Hepatitis:
- Short drinking binge → acute;
- chronic heavy drinking → chronic hepatitis.
- Toxic Hepatitis: Caused by drugs and industrial chemicals.
- Autoimmune Hepatitis: Immune system mistakenly attacks the liver’s own cells.
- Other causes: Cholestasis (reduced bile flow), inherited metabolic disorders, ischemia (restricted blood flow).
6. Zika Virus Disease
Zika has been in the news quite regularly, and its connection to pregnancy makes it particularly important from a public health perspective. It was first discovered in Uganda’s Zika Forest in 1947.
| Aspect | Details |
| Causative Agent | Zika virus — a Flavivirus (mosquito-borne) |
| Transmission | 1. Aedes mosquito bites (daytime biting) 2. Mother-to-child (during pregnancy) 3. Sexual transmission (unprotected sex) |
| Symptoms | Rash, fever, conjunctivitis, muscle/joint pain, malaise — most infections are ASYMPTOMATIC |
| Complications | Microcephaly (small head in newborns), Guillain-Barré Syndrome, Myelitis (spinal cord inflammation), Peripheral neuropathy |
| Treatment/Prevention | NO vaccine or specific treatment available |
Microcephaly — Remember This! Microcephaly is a neurological condition where a baby’s head is significantly smaller than normal, often associated with brain damage. Zika’s link to microcephaly made it a global health emergency in 2016.
7. Dengue
Dengue, nicknamed ‘breakbone fever’ because of the excruciating bone and joint pain it causes, is one of India’s most prevalent mosquito-borne diseases. It is caused by the Dengue virus, a Flavivirus transmitted by the Aedes mosquito.
Transmission
- Primary vector: Infected female Aedes mosquitoes — they bite mainly during the daytime (this distinguishes them from malaria-spreading Anopheles).
- Note: Aedes also transmits Chikungunya, Yellow Fever, and Zika.
- Mother-to-child transmission during pregnancy and childbirth is also possible.
Complications — The Dangerous Part
- Severe Dengue / Dengue Haemorrhagic Fever (DHF): The dengue virus damages platelets and blood vessels, causing plasma leakage — internal and external bleeding. Symptoms include stomach pain, vomiting blood, nosebleeds, and bleeding gums.
- Dengue Shock Syndrome: Severe plasma leakage leads to low blood pressure and inadequate blood supply to vital organs — can cause organ failure and death if untreated.
- Thrombocytopenia: Drastic fall in platelet count due to
- (a) bone marrow suppression — the virus suppresses platelet production, and
- (b) immune-mediated destruction — antibodies triggered by the infection attack platelets.
Treatment & Prevention: No medicine cures dengue — only supportive care and symptom management.
Two dengue vaccines are licensed globally but are UNAVAILABLE in India due to inadequate safety data.
8. Nipah
Nipah has a special relevance for India — particularly Kerala — where multiple outbreaks have occurred. It is a zoonotic disease caused by the Nipah virus, an RNA virus.
| Aspect | Details |
| Causative Agent | Nipah virus (RNA virus) |
| Natural Host | Fruit bats (flying foxes) of the Pteropodidae family |
| Transmission | Animal-to-human (esp. pigs), Human-to-human (close contact/bodily fluids), Foodborne (contaminated food) |
| Symptoms | Acute respiratory illness, seizures, encephalitis (brain swelling) |
| Fatality Rate | 40–75% of infected cases — one of the highest case fatality rates |
| Treatment/Prevention | No vaccine or antiviral treatment available; only symptomatic management |
India Connect Nipah virus is endemic in bats in Kerala. Outbreaks occur in human habitations close to forests, increasing exposure to infected bats and host animals.
Kerala has reported multiple Nipah outbreaks (2018, 2019, 2021, 2023).
9. Influenza (Flu)
Influenza is a contagious respiratory illness caused by influenza viruses. It infects the nose, throat, and sometimes the lungs. In temperate climates, it primarily peaks in winter; in tropical regions like India, it can occur year-round.
Transmission
- Droplets: Respiratory droplets from coughs, sneezes, or talking.
- Fomites: Touching contaminated surfaces, then touching the face/mouth/nose.
- Direct contact: Touching infected individuals.
High-Risk Groups
Pregnant women, young children under 5, elderly (65+), people with chronic illnesses (asthma, diabetes, heart disease), and immunocompromised individuals (cancer patients, HIV-positive people).
Types of Influenza Viruses
| Type | Who It Infects | Severity | Pandemic Risk | Special Feature |
| Influenza A | Humans, birds, pigs (cross-species) | Most Severe | YES — only type to cause global pandemics | Subtypes by H (18) and N (11) proteins |
| Influenza B | Primarily humans | Less severe | Unlikely (no subtypes) | Two lineages: B/Yamagata and B/Victoria |
| Influenza C | Humans, some pigs | Mild (like common cold) | No | Does not cause epidemics |
| Influenza D | Primarily cattle | Not known to infect humans | No | Recently identified; under study |
Remember:
H1N1 = Swine Flu (Influenza A). H5N1 = Avian/Bird Flu (Influenza A).
Wild aquatic birds are the primary natural reservoir for Influenza A. Annual flu vaccination is needed because influenza viruses mutate rapidly.
10. Monkeypox (Mpox)
Monkeypox gained global attention in 2022 when it was declared a Public Health Emergency of International Concern (PHEIC) by WHO. It is caused by the Monkeypox virus, an orthopoxvirus — the same genus to which the notorious smallpox virus belongs.
| Aspect | Details |
| Causative Agent | Monkeypox virus — orthopoxvirus (same genus as smallpox virus) |
| Subtypes/Variants | Clade I and Clade II |
| Natural Reservoir | Unknown; rodents (rope squirrels, tree squirrels, Gambian pouched rats) and non-human primates are susceptible |
| Transmission | Animal-to-human: bites, scratches, bush meat. Human-to-human: skin lesions, bodily fluids, respiratory droplets, sexual contact, contaminated objects |
| Symptoms | Fever, muscle ache, rash, swollen lymph nodes, pus-filled skin lesions, chills, exhaustion |
| Fatality Rate | 0.1–10% of infected individuals |
| High-Risk Groups | Pregnant women, children, immunocompromised persons |
| Treatment/Prevention | No specific treatment. Vaccines available; smallpox vaccines also effective against Mpox |
11. Marburg Virus Disease (MVD)
Marburg is often called the ‘sister disease’ to Ebola — and for good reason. Both are caused by viruses of the Filoviridae family, both cause haemorrhagic fever, and both are terrifyingly lethal. Marburg has a fatality rate of up to 88%.
| Aspect | Details |
| Causative Agent | Marburg virus — Filoviridae family (same as Ebola) |
| Natural Host | Rousettus aegyptiacus (Egyptian fruit bats) |
| Transmission | Animal-to-human: exposure to infected bats (saliva, urine, faeces). Human-to-human: direct contact with blood or bodily fluids |
| Fatality Rate | Up to 88% |
| Complications | Haemorrhagic fever, jaundice, organ failure (liver/kidney), encephalitis, delirium |
| Treatment/Prevention | No antiviral treatment or vaccine currently available |
12. COVID-19
COVID-19 needs no introduction — it reshaped the entire world between 2020 and 2023. It is caused by SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2), first identified in Wuhan, China in 2019. It was declared a global pandemic by WHO in March 2020.
What are Coronaviruses?
Coronaviruses are a family of RNA viruses. They are called ‘corona’ because of the crown-like (corona = crown) spike proteins on their surface. Examples include SARS, MERS, and the common cold virus.
Variants of SARS-CoV-2
Several variants emerged during the pandemic — Alpha, Beta, Delta, and Omicron — each differing in transmissibility, severity, and ability to escape immune responses (immune escape).
Transmission
- Respiratory droplets: Coughing, sneezing, talking, breathing.
- Direct contact: Handshakes with infected persons.
- Fomites: Touching contaminated surfaces (door handles, phones).
Key Complications
- Pneumonia, Acute Respiratory Distress Syndrome (ARDS), Hypoxemia (low blood oxygen levels), blood clots.
- Black Fungus (Mucormycosis): A rare but serious fungal infection caused by mucormycetes fungi found in soil and decaying matter. COVID-19 induced immunosuppression + steroid treatment + underlying diabetes increased susceptibility.
- Long COVID: Continuation or development of new symptoms 3 months after initial infection, lasting at least 2 months. Common symptoms include fatigue, shortness of breath, and cognitive dysfunction (brain fog).
COVID-19 Vaccines
| Vaccine | Type | Developer | Key Fact |
| Covishield | Viral Vector | Oxford-AstraZeneca (made by SII in India) | Uses chimpanzee adenovirus vector |
| Covaxin | Inactivated Pathogen | Bharat Biotech + ICMR + NIV | India’s first indigenous COVID vaccine |
| Sputnik V | Viral Vector | Gamaleya Institute, Russia | Uses two different human adenoviruses |
| Pfizer-BioNTech | mRNA-based | Pfizer + BioNTech | First ever approved mRNA vaccine |
| Moderna | mRNA-based | Moderna | mRNA platform |
Vaccine Technology: mRNA-based vaccines use genetically engineered mRNA to instruct cells to produce a spike protein, triggering immunity. Viral vector vaccines use a harmless virus to deliver instructions for making antigens.
13. Malaria
Malaria is one of the oldest known infectious diseases — it has plagued civilisations for millennia. Unlike most diseases in this chapter (which are viral), Malaria is caused by a protozoan parasite — specifically of the Plasmodium group. This makes it unique and important to remember.
Five Species of Plasmodium That Infect Humans
| Species | Severity & Geography | Special Note |
| P. falciparum | Most severe; common in Africa | Drug-resistant mutations observed; can relapse |
| P. vivax | Severe; common in Asia & Latin America | Can cause relapses |
| P. ovale | Less severe | Relapses less frequently |
| P. malariae | Milder malaria | — |
| P. knowlesi | Zoonotic; found in macaques (old-world monkey) of Southeast Asia | Transmitted from monkeys to humans |
Most Dangerous Duo: P. falciparum and P. vivax pose the greatest threat to human health globally.
Transmission & Treatment
Malaria is transmitted through the bites of infected female Anopheles mosquitoes. The parasites first multiply in the liver, then destroy red blood cells (RBCs), causing the characteristic fever, chills, and yellow skin (jaundice).
Unlike most diseases in this section, malaria is CURABLE with antimalarial drugs.
- Artemisinin-Based Combination Therapies (ACTs): First-line treatment for P. falciparum malaria.
- Chloroquine: Used for P. vivax, P. ovale, and P. malariae — but ineffective against chloroquine-resistant P. falciparum.
- Quinine: Used in severe malaria cases.
Vaccines:
WHO recommends two vaccines: RTS,S/AS01 (Mosquirix) and R21/MatrixM.
Neither is currently available in India.
14. Tuberculosis (TB)
India carries the highest TB burden in the world. TB is caused by the bacterium Mycobacterium tuberculosis of the Mycobacteriaceae family.
Interestingly, leprosy is caused by another member of the same family — Mycobacterium leprae.
Types of TB
- Latent TB: The person is infected but shows NO symptoms and cannot spread the disease. It can become active if untreated.
- Active TB: Bacteria are multiplying, causing symptoms. The person can transmit the disease to others.
- Pulmonary TB: Affects the lungs — the most common form.
- Extra-pulmonary TB: Affects other organs — kidneys, bones, lymph nodes, spine, and brain.
Drug Resistance — A Major UPSC Topic
| Type | Resistant To | Cause |
| MDR-TB (Multi-Drug Resistant) | Isoniazid + Rifampicin (the two most powerful first-line drugs) | Treatment mismanagement or person-to-person transmission |
| XDR-TB (Extensively Drug Resistant) | Isoniazid + Rifampicin + any fluoroquinolone + at least one injectable second-line drug | Rare form arising from MDR-TB |
Treatment & Prevention
Standard TB treatment involves a combination of antibiotics over 6–9 months, including Isoniazid, Rifampin, Ethambutol, and Pyrazinamide.
MDR-TB and XDR-TB require special treatment regimens. Prevention is through the BCG vaccine (Bacillus Calmette-Guérin), given to infants and children in India.
India Target: India aims to eliminate TB by 2025 — five years ahead of the global target set under the UN Sustainable Development Goals (SDGs).
15. Diabetes
Diabetes mellitus is a chronic, progressive non-communicable disease. Unlike all the previous diseases in this section which are communicable infections, diabetes is a metabolic disorder.
It is characterised by high blood glucose (Hyperglycaemia) — either because the pancreas does not produce enough insulin, or the body cannot effectively use the insulin it produces.
Insulin’s Role: Insulin is a hormone produced by the pancreas that acts like a ‘key’ to let glucose enter the body’s cells for energy. Without sufficient or effective insulin, glucose accumulates in the blood — this is Hyperglycaemia.
Types of Diabetes
| Type | Cause | Age of Onset | % of Diabetics | Management |
| Type 1 | Immune system destroys insulin-producing pancreatic cells | Childhood/Adolescence | 5–10% | Insulin injections (lifelong) |
| Type 2 | Insulin resistance or insufficient production | Adults (most common) | 90–95% | Lifestyle changes, oral medications, sometimes insulin |
| Type 1.5 (LADA) | Autoimmune; attacks beta cells — slower than Type 1 | Adults (usually 30+) | Small % | Oral meds initially; insulin eventually |
| Gestational | Body cannot produce enough insulin during pregnancy | 2nd–3rd trimester of pregnancy | ~ | Diet, exercise, sometimes insulin |
Complications of Uncontrolled Diabetes
- Heart disease and stroke
- Kidney damage — Diabetic Nephropathy
- Nerve damage — Neuropathy
- Eye damage — Retinopathy
- Poor wound healing and increased infection susceptibility
16. West Nile Virus (WNV)
West Nile Virus is a mosquito-borne RNA virus belonging to the Flavivirus genus — same family as dengue, Zika, and yellow fever. It infects humans, birds, and animals, but birds are the reservoir hosts.
| Aspect | Details |
| Causative Agent | West Nile Virus — Flavivirus (RNA virus) |
| Reservoir Host | Birds |
| Transmission | Primarily via infected Culex mosquitoes; rarely through blood transfusion, organ transplant, or mother-to-child |
| Symptoms | 80% asymptomatic. Mild: fever, headache, fatigue, rash. Severe: encephalitis, meningitis, paralysis, coma |
| Treatment/Prevention | No specific antiviral treatment or vaccine available for humans |
17. Elephantiasis (Lymphatic Filariasis)
Elephantiasis is a neglected tropical disease — a term used for diseases that predominantly affect the poorest populations in tropical regions. The name comes from the characteristic massive swelling of limbs that resembles an elephant’s skin.
| Aspect | Details |
| Causative Agent | Parasitic worms: Wuchereria bancrofti (most common), Brugia malayi, Brugia timori |
| Transmission | Bites of infected mosquitoes — mainly Culex, Anopheles, and Aedes species |
| Affected Areas | Lymphatic system — particularly legs, arms, breasts, and genitals |
| Symptoms | Swelling of limbs (lymphedema), thickening of skin (elephantiasis), genital swelling, pain, fever |
| Treatment | Not curable, but manageable with antiparasitic medications |
| Prevention | Mosquito control and Mass Drug Administration (MDA) with antiparasitic medications |
India Target: India aims to end Lymphatic Filariasis by 2027 — three years ahead of the global target.