Viral Diseases

NCERT grounding

NCERT Class XII Chapter 7, in its survey of common infectious diseases, devotes a single dense paragraph to viruses, anchored on Rhinoviruses as the agent of the common cold. NIOS Senior Secondary Biology, Module 4, Lesson 29.3.1 (Diseases caused by viruses) supplements this with a structured roster — chicken pox, measles, poliomyelitis, rabies, hepatitis B, influenza and dengue — each given a pathogen-mode-incubation-symptom-control profile. Recent NCERT updates also reference COVID-19 as a coronavirus pandemic in the chapter introduction, and Aedes-borne dengue and chikungunya are cited in the prevention section.

"Rhino viruses represent one such group of viruses which cause one of the most infectious human ailments — the common cold. They infect the nose and respiratory passage but not the lungs."

NCERT Class XII, Chapter 7, Section 7.1

Two examiner habits flow from this grounding. First, NEET memorises NCERT verbatim: Rhinoviruses appear in stems exactly as "Rhino virus" or "Rhinoviruses," and the phrase "but not the lungs" is itself a trap because students confuse cold with pneumonia. Second, NEET pulls supplementary names (influenza, dengue, measles) from NIOS-level coverage, where they exist as recognition items. A focused subtopic page must therefore drill the common cold and influenza pair to depth and treat the rest at agent-plus-transmission accuracy.

The NEET viral disease roster

Across the 2016–2025 NEET papers, viral disease items have appeared as the "A" row of List-I/List-II matching questions (2024 Q.177 pairs common cold to Rhinoviruses) and as standalone agent-identification stems. The roster below collects every NCERT- or NIOS-named virus that has surfaced or that an updated edition references; the depth signal in column three guides how to study each.

Common cold — Rhinovirus

The common cold is the headline viral entry in NCERT Chapter 7. The causative agent is the Rhinovirus — a small, non-enveloped RNA virus of the family Picornaviridae. Over a hundred serotypes circulate in human populations, which is why immunity to one bout does not protect against the next and why no commercial vaccine exists. NCERT confines the disease to the upper respiratory tract: Rhinoviruses "infect the nose and respiratory passage but not the lungs," a phrase exam-setters lift directly into stems to catch students who conflate cold with pneumonia or influenza.

Symptoms

NCERT lists six symptoms in a single sentence: nasal congestion and discharge, sore throat, hoarseness, cough, headache and tiredness. These persist for roughly 3–7 days and resolve without antiviral therapy. The infection is self-limiting because innate interferon responses and adaptive antibody production clear most rhinovirus strains within a week.

Transmission

Two transmission routes are explicit in NCERT. Droplets generated by cough or sneezes are inhaled directly by a healthy contact, and contaminated objects — pens, books, cups, doorknobs, computer keyboards or computer mice — serve as fomites that carry the virus across surfaces. The fomite route is a frequent NEET hook because the object list is so specific.

Influenza — Influenza virus

Influenza, commonly called the "flu," is caused by the Influenza virus, a segmented RNA virus of the Orthomyxoviridae family with three principal types — A, B and C. NIOS Module 4 ranks influenza as a more severe respiratory illness than the common cold, with a distinct clinical signature and an annual vaccine programme. NEET treats influenza as the second core viral entry, especially through statement-pair items and recognition stems.

Clinical signature

Typical influenza symptoms include high fever (usually 100–103°F in adults, often higher in children), cough, sore throat, running nose, headache, muscle pain (myalgia) and extreme fatigue. Nausea, vomiting and diarrhoea can accompany the infection, especially in children, but gastrointestinal symptoms are rarely prominent. Most people recover in 1–2 weeks; complications can include bacterial pneumonia, which is the chief cause of influenza-associated death.

Antigenic drift and the vaccination programme

The influenza virus mutates continuously, producing new variants — a process called antigenic drift. Antibodies raised against one strain do not recognise the new mutated coat proteins, which is why influenza recurs and why a single childhood vaccine does not confer life-long immunity. NIOS states the vaccine recommendation directly: an annual influenza vaccination can prevent much of the illness and death, particularly in patients with chronic heart, lung or kidney disease, diabetes, or severe anaemia.

AIDS — pointer to the sibling page

AIDS (Acquired Immuno Deficiency Syndrome) is the most heavily examined viral disease in NEET, but its mechanism is far richer than the matching items the rest of this roster supports. Because of this, AIDS sits on its own dedicated NEETgrid page. The headline facts to remember in this subtopic are these: AIDS is caused by the Human Immunodeficiency Virus (HIV), a member of a group of viruses called retroviruses that carry an RNA genome inside an envelope. HIV preferentially infects helper T-lymphocytes (TH cells), replicates within them and progressively destroys the cell-mediated immune compartment. Confirmed by ELISA, treated by anti-retroviral therapy, and transmitted by sexual contact, contaminated blood/needles and mother-to-child routes. For the full mechanism, replication cycle figure and PYQ cluster (2023 Q.166 on HIV replication in TH cells, for example), follow the sibling link in the related-topics block at the foot of this page.

Measles and mumps

Measles and mumps are the two classic childhood viral diseases that sit just outside NCERT's named roster but show up in NIOS Lesson 29 and the standard NEET vocabulary. Both are prevented by the MMR vaccine (measles, mumps, rubella) and both transmit through droplets.

Arboviral fevers — dengue and chikungunya

NCERT closes the prevention section of Chapter 7 with an explicit mention of Aedes mosquitoes as the vector of dengue and chikungunya in India. NIOS Lesson 29.3.1 then gives dengue a full clinical workup. NEET treats this pair as the standard "mosquito-borne viral fever" cluster, distinguishing it from the Anopheles–Plasmodium pair (malaria) and the Culex–Wuchereria pair (filariasis).

Dengue

Dengue is an acute fever caused by the dengue virus (DENV), transmitted by the bite of an infected Aedes aegypti mosquito. Aedes rests indoors in dark spaces, bites mostly in daytime, and lays its eggs in stagnant water in coolers, tyres and discarded containers — a vector ecology NEET probes through prevention items. The incubation period between bite and first symptoms averages 4–6 days (range 3–14). Sudden high fever (104–105°F), severe forehead headache, joint and muscle pain, and characteristic retro-orbital pain (pain behind the eyes that worsens with eye movement) are the classical signature, hence the older name "break-bone fever."

Dengue haemorrhagic fever is the advanced and dangerous form, adding bleeding (skin petechiae, gum and nose bleeds, internal bleeding, black stools) and abdominal pain. Diagnosis combines serology (antibodies against dengue viruses) with a sharp fall in platelet count, which is the lab marker most often referenced in test stems. There is no specific antiviral therapy; treatment is supportive — fluid replacement, rest, and blood transfusion in haemorrhagic cases.

Chikungunya

Chikungunya is caused by the chikungunya virus, also vectored by Aedes mosquitoes. Symptoms — high fever and severe joint pain that can persist for weeks or months — overlap with dengue, but platelet counts are typically less depressed and the joint pain is the dominant complaint. NCERT cites chikungunya alongside dengue in the Aedes-vector paragraph; that single mention is the depth NEET asks for.

H1N1 swine flu and COVID-19

Two pandemic-era viral diseases now sit in the NEET-aware updated NCERT corpus: H1N1 swine flu and COVID-19. Both are respiratory illnesses, both transmit by droplets, and both are recognition items rather than mechanism items at the NEET level.

H1N1 (swine flu) is a strain of Influenza A virus that crossed from pigs to humans and caused the 2009 pandemic. Clinically it resembles seasonal influenza — high fever, cough, sore throat, body ache, fatigue — but spreads more rapidly through naive populations. The H1N1 surface antigen names "hemagglutinin 1, neuraminidase 1" are the spelt-out reason the strain identifier looks like an HnNn code. Annual influenza vaccines now include H1N1 lineage strains.

COVID-19 is caused by SARS-CoV-2, a coronavirus first reported in late 2019. NCERT updated editions reference the disease as the current global pandemic context. The virus spreads primarily by respiratory droplets and aerosols during coughing, sneezing or talking, and via contaminated surfaces. Symptoms range from mild fever, dry cough and loss of taste/smell to severe pneumonia and acute respiratory distress requiring oxygen. Prevention relies on the combination of vaccination (mRNA, viral-vector and inactivated platforms), mask use, hand hygiene and physical distancing — a public-health response NCERT cites as the modern parallel to smallpox eradication.

Prevention and control across the viral roster

NCERT closes Section 7.1 with a unified prevention frame that applies cleanly across the viral roster. For air-borne viral diseases (common cold, influenza, measles, mumps, COVID-19, H1N1), the additional measure beyond general hygiene is to avoid close contact with infected persons or their belongings. For vector-borne viral diseases (dengue and chikungunya via Aedes), the priority is to control or eliminate the vector and its breeding places — no stagnant water in coolers and tyres, mosquito nets, Gambusia fish in ponds, insecticide spray in ditches, and wire-mesh windows.

Vaccines deserve particular emphasis. NCERT explicitly notes that smallpox has been eradicated through immunisation, and that polio, diphtheria, pneumonia and tetanus have been controlled to a large extent through vaccine programmes. Among viral diseases, polio is on the brink of global eradication, the MMR vaccine covers measles and mumps, hepatitis B vaccine is integrated into national programmes, and the COVID-19 vaccine roll-out has been the largest immunisation campaign in human history.

Worked examples

Common confusion & NEET traps