Which one of the following diseases is caused by virus ?

1. Classification of Pathogenic Microorganisms (basic)

To understand human infectious diseases, we must first meet the "invisible" actors behind them. Microorganisms (or microbes) are living entities so small they cannot be seen without a microscope. While they exist everywhere—in soil, water, and even inside our bodies—they are not a single uniform group. Instead, they are classified into distinct categories based on their structure and biological behavior: Bacteria, Fungi, Protozoa, and some Algae Science, Class VIII NCERT, Chapter 3, p.16. While many microbes are beneficial—such as those helping in digestion or fixing nitrogen in soil—those that cause diseases are specifically termed pathogens.

The biological differences between these groups are significant. For instance, Bacteria are single-celled organisms that lack a well-defined nucleus but possess a protective cell wall Science, Class VIII NCERT, Chapter 3, p.24. In contrast, Protozoa like Amoeba or Paramecium are often more complex single cells that can move independently Science, Class VIII NCERT, Chapter 3, p.16. Fungi, such as yeast or molds, can be single-celled or multicellular and play a major role in decomposition Science, Class VIII NCERT, Chapter 3, p.25. Understanding these differences is vital because the "medicine" used to kill a bacterium (antibiotics) usually has no effect on a virus or a fungus due to these structural variations.

Viruses occupy a unique and somewhat controversial place in this classification. They are significantly smaller than bacteria and do not show typical signs of life on their own. They are considered to be on the borderline between living and non-living because they reproduce only inside the cells of a host organism (be it a plant, animal, or bacterium) Science, Class VIII NCERT, Chapter 3, p.24. Once they enter a host, they hijack the host's cellular machinery to multiply, often causing disease in the process.

Category	Key Characteristics	Examples
Bacteria	Single-celled; no defined nucleus; have cell walls.	Rhizobium, Lactobacillus
Viruses	Ultramicroscopic; reproduce only inside host cells.	Influenza virus, COVID-19
Protozoa	Single-celled; often irregular shapes; capable of movement.	Amoeba, Paramecium
Fungi	Can be unicellular or multicellular; lack chlorophyll.	Yeast, Bread mould

Sources: Science, Class VIII NCERT, Chapter 3: Health: The Ultimate Treasure, p.16; Science, Class VIII NCERT, Chapter 3: Health: The Ultimate Treasure, p.24; Science, Class VIII NCERT, Chapter 3: Health: The Ultimate Treasure, p.25

2. Structural Differences: Bacteria vs. Viruses (intermediate)

To understand human infectious diseases, we must first distinguish between the two most common pathogens: bacteria and viruses. At a fundamental level, bacteria are complex, single-celled living organisms that can survive on their own in diverse environments, from deep-sea vents to the human gut. In contrast, viruses are much smaller and simpler, consisting essentially of genetic material (DNA or RNA) encased in a protein shell called a capsid.

Structurally, bacteria are prokaryotic cells. They possess a cell membrane and most have a protective cell wall, but they "lack a well-defined nucleus" (Science Class VIII, The Invisible Living World, p.24). Viruses, however, do not have a cellular structure at all. They lack the machinery for metabolism and do not show "molecular movement" unless they have successfully infected a host cell (Science Class X, Life Processes, p.79). This leads to a famous biological debate: while bacteria are clearly alive, viruses exist in a grey area between living and non-living matter.

The most critical functional difference lies in how they replicate. Bacteria are independent and reproduce through simple cell division. Viruses are obligate intracellular parasites, meaning they "reproduce only inside the host organism" (Science Class VIII, The Invisible Living World, p.24). They must hijack a host's cellular machinery to create copies of themselves. This is why antibiotics are effective against bacteria but useless against viruses; antibiotics are designed to target specific bacterial structures, like the cell wall or metabolic pathways, that viruses simply do not possess (Science Class VIII, Health: The Ultimate Treasure, p.39).

Feature	Bacteria	Viruses
Type	Single-celled living organism	Non-cellular genetic particle
Size	Larger (Visible under light microscope)	Submicroscopic (Much smaller)
Reproduction	Independent (Fission)	Requires a host cell to replicate
Treatment	Antibiotics	Vaccines and Antivirals (Antibiotics fail)

Sources: Science Class VIII (NCERT 2025), The Invisible Living World: Beyond Our Naked Eye, p.24; Science Class X (NCERT 2025), Life Processes, p.79; Science Class VIII (NCERT 2025), Health: The Ultimate Treasure, p.39

3. Modes of Disease Transmission (basic)

Hello! Now that we understand what infectious diseases are, let’s look at the mechanics of their movement. For a pathogen (like a virus or bacteria) to cause an outbreak, it must travel from an infected host or environment to a healthy person. This pathway is known as the Mode of Transmission. Understanding these modes is the bread and butter of public health because if we can block the pathway, we can stop the disease.

Transmission is generally classified into two broad categories: Direct and Indirect. In Direct Transmission, the pathogen transfers immediately through physical contact. This includes skin-to-skin touch or droplet spread, where heavy droplets from a cough or sneeze travel a short distance (usually less than a meter) to land on someone's eyes, nose, or mouth Geography of India, Majid Husain, Contemporary Issues, p.25. A specialized form of direct transmission is through intimate sexual contact, which is the primary route for infections like HIV-AIDS or Syphilis Science, Class X NCERT, How do Organisms Reproduce?, p.125.

Indirect Transmission is more complex because it involves an intermediate “carrier.” These carriers fall into three main groups:

• Vehicle-borne: Pathogens travel through contaminated non-living substances like water and food. Diseases like Typhoid and Cholera are classic examples of water-borne transmission Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.37.
• Vector-borne: Living organisms, typically arthropods like mosquitoes or flies, carry the germ. Mosquitoes are particularly effective transmitters for diseases like Malaria or Dengue Geography of India, Majid Husain, Contemporary Issues, p.25.
• Airborne: Unlike droplets, these are tiny particles that remain suspended in the air for long periods and can travel long distances.

Interestingly, environmental changes can shift these patterns. For instance, climate change and rising temperatures can expand the transmission window for vector-borne diseases like malaria, allowing mosquitoes to survive in regions they couldn't before Environment, Shankar IAS Academy, India and Climate Change, p.310.

Mode	Mechanism	Examples
Direct Contact	Touching, kissing, or droplets from a sneeze.	Common Cold, Skin infections.
Water-borne	Ingesting contaminated water/food.	Typhoid, Cholera, Jaundice.
Vector-borne	Carried by insects (mosquitoes, flies).	Malaria, Dengue, Chikungunya.

Sources: Geography of India, Majid Husain, Contemporary Issues, p.25; Science, Class X NCERT, How do Organisms Reproduce?, p.125; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.37; Environment, Shankar IAS Academy, India and Climate Change, p.310

4. India's Universal Immunization Programme (UIP) (intermediate)

The Universal Immunization Programme (UIP) is one of the most vital public health initiatives in India and one of the largest such programmes in the world. Its primary goal is to provide life-saving vaccines to infants, children, and pregnant women to prevent a range of debilitating diseases. It is rooted in the principle that vaccines are preventive, not curative; they work by preparing the immune system to fight off infections before they cause serious illness Science, Class VIII NCERT (Revised ed 2025), Chapter 3, p.39. Originally launched as the 'Expanded Programme on Immunization' (EPI) in 1978, it was redesigned as the UIP in 1985 to achieve self-sufficiency in vaccine production and expand coverage across the nation.

The UIP currently provides protection against 12 vaccine-preventable diseases (VPDs). Nationally, it covers 11 diseases: Tuberculosis, Diphtheria, Pertussis (Whooping Cough), Tetanus, Polio, Hepatitis B, Pneumonia and Meningitis caused by Haemophilus Influenzae type b (Hib), Measles, Rubella, Japanese Encephalitis (JE), and Rotavirus diarrhoea. A 12th vaccine, the Pneumococcal Conjugate Vaccine (PCV), is also part of the schedule. It is important to distinguish between these; for instance, while Tuberculosis and Diphtheria are bacterial, others like Polio and Measles are viral Science, Class VIII NCERT (Revised ed 2025), Chapter 3, p.33-34. This mix of coverage ensures a comprehensive shield against various pathogens.

To address gaps in coverage and reach the 'unreached' populations, the Government of India launched Mission Indradhanush in 2014. This strategic intervention aims to achieve at least 90% full immunization coverage across India. The programme emphasizes that getting vaccinated not only protects the individual but also creates herd immunity, protecting the wider community, including those who cannot be vaccinated for medical reasons Science, Class VIII NCERT (Revised ed 2025), Chapter 3, p.39.

Milestone	Year	Significance
EPI Launched	1978	Initial focus on urban immunization.
UIP Renamed	1985	Shifted focus to universal coverage and rural reach.
Mission Indradhanush	2014	Intensive drive to reach children missed by routine services.

Sources: Science, Class VIII NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.39; Science, Class VIII NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.33-34

5. Antimicrobial Resistance (AMR) and Superbugs (exam-level)

To understand Antimicrobial Resistance (AMR), we must first look at the breakthrough that changed modern medicine: the discovery of antibiotics. In 1928, Alexander Fleming noticed that a mould (Penicillium notatum) on a petri dish killed surrounding bacteria, leading to the creation of Penicillin Science, Class VIII NCERT, Chapter 3, p.40. Antibiotics are chemical substances produced by microorganisms that can kill or stop the growth of disease-causing bacteria. While they have saved millions of lives, their indiscriminate use has triggered a global health crisis where bacteria evolve to survive these drugs.

AMR occurs when microorganisms (like bacteria, viruses, or fungi) change over time and no longer respond to medicines, making infections harder to treat. When bacteria specifically become resistant to most of the antibiotics commonly used today, they are termed Superbugs. This isn't just a biological accident; it is evolution in action. When a patient takes the wrong dose or stops a course midway, the "weakest" bacteria die, but the "stronger" ones with natural mutations survive and multiply Science, Class VIII NCERT, Chapter 3, p.41. These resistant traits are then passed on, rendering the antibiotic ineffective for future use.

Factor	Contribution to Resistance
Misuse	Taking antibiotics for viral infections like the common cold (antibiotics only work on bacteria).
Incomplete Course	Stopping medication early allows surviving bacteria to develop and spread resistance.
Agricultural Use	Using antibiotics to promote growth in livestock creates a reservoir of resistant bacteria that can enter the human food chain.

To tackle this, global and national policies emphasize Antibiotic Stewardship — using these drugs only when prescribed by a doctor, in the correct dose, and for the full duration Science, Class VIII NCERT, Chapter 3, p.41. In India, while the pharmaceutical industry produces a wide range of formulations through units like Karnataka Antibiotics and Pharmaceuticals Ltd Geography of India, Majid Husain, Chapter 13, p.61, there is an increasing push to integrate traditional knowledge systems like Ayurveda and Unani to manage health and reduce the heavy reliance on synthetic antibiotics for minor ailments Science, Class VIII NCERT, Chapter 3, p.41.

Sources: Science, Class VIII NCERT, Health: The Ultimate Treasure, p.40; Science, Class VIII NCERT, Health: The Ultimate Treasure, p.41; Geography of India, Majid Husain, Industries, p.61

6. Zoonotic and Emerging Viral Threats (exam-level)

To understand the landscape of modern health, we must start with Zoonoses — infectious diseases that jump from animals to humans. These spillover events are not just biological accidents; they are often the result of human encroachment into wildlife habitats. While many diseases are communicable, it is vital to distinguish between pathogens. For instance, while Tuberculosis and Anthrax are caused by bacteria, threats like Rabies and Influenza are viral in nature Environment, Shankar IAS Academy, p.193. Viral threats are particularly concerning because they often mutate rapidly, allowing them to adapt to human hosts and potentially trigger global pandemics.

Emerging viral threats often follow a pattern of host-switching. Many viruses reside harmlessly in wild reservoirs, such as bats or primates, but become dangerous when they jump to humans. For example, Rabies affects a wide range of Indian wildlife, from tigers to squirrels, while Foot and Mouth Disease (FMD) impacts wild ungulates like Gaur and Nilgai Environment, Shankar IAS Academy, p.193. Identifying these viruses in the wild is the first step in pandemic prevention.

Globally, the focus has shifted toward a "One Health" approach — the idea that human health, animal health, and environmental health are inextricably linked. This is exemplified by international programs like PREZODE, which was launched to prevent the emergence of zoonotic diseases by protecting biodiversity and monitoring high-risk regions Environment, Shankar IAS Academy, p.348. By protecting 30% of the planet's land and sea, we create a buffer that reduces the frequency of human-wildlife contact, thereby lowering the risk of the next viral spillover.

Disease Type	Common Examples	Pathogen Type
Zoonotic Viral	Rabies, Influenza, FMD	Virus
Zoonotic Bacterial	Anthrax, Tuberculosis	Bacteria
Human-Specific	Typhoid, Diphtheria	Bacteria

Sources: Environment, Shankar IAS Academy, Animal Diversity of India, p.193; Environment, Shankar IAS Academy, Climate Change Organizations, p.348; Science, Class VIII, NCERT, Health: The Ultimate Treasure, p.33

7. Common Bacterial Diseases: TB, Typhoid, and Diphtheria (intermediate)

In our journey through human infectious diseases, we now focus on Bacterial Diseases. Unlike viruses, which are essentially genetic material in a protein coat, bacteria are complex, single-celled organisms that can survive in diverse environments. In the context of the UPSC syllabus, three major bacterial infections—Tuberculosis, Typhoid, and Diphtheria—are critical because of their public health impact and specific modes of transmission.

1. Tuberculosis (TB): Caused by the bacterium Mycobacterium tuberculosis, this disease primarily targets the lungs, though it can affect other parts of the body like the spine or kidneys. It is a highly contagious respiratory disease spread through droplet nuclei when an infected person coughs or sneezes Geography of India, Majid Husain, Contemporary Issues, p.25. Common symptoms include a persistent cough (sometimes with blood), night sweats, and significant weight loss Science, Class VIII NCERT, Chapter 3, p.34. Interestingly, various species of Mycobacterium also affect wildlife, including deer and elephants Environment, Shankar IAS Academy, Chapter 12, p.193.

2. Typhoid: This is a classic example of a water-borne disease. It is caused by Salmonella Typhi. Unlike TB, which is airborne, Typhoid spreads through the fecal-oral route—meaning it is transmitted via contaminated food and water Science, Class VIII NCERT, Chapter 3, p.34. It affects the intestines, leading to sustained high fever, extreme fatigue, headache, and abdominal pain. Maintaining personal hygiene and ensuring access to boiled drinking water are the primary preventive measures.

3. Diphtheria: This is an acute infection caused by Corynebacterium diphtheriae. It typically affects the mucous membranes of the nose and throat. A hallmark of this disease is the formation of a thick, grayish membrane in the throat, which can make breathing and swallowing difficult. If left untreated, the bacteria release toxins that can enter the bloodstream and cause severe damage to the heart and nerves. Prevention is highly effective through the DPT (Diphtheria, Pertussis, and Tetanus) vaccine.

To help you distinguish between these for your revision, look at their primary targets and modes of spread:

Disease	Pathogen	Primary Target	Transmission
Tuberculosis	Mycobacterium tuberculosis	Lungs	Airborne Droplets
Typhoid	Salmonella Typhi	Intestines	Contaminated Food/Water
Diphtheria	Corynebacterium diphtheriae	Throat/Respiratory Tract	Droplets/Direct Contact

Sources: Science, Class VIII NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.34; Environment, Shankar IAS Academy (ed 10th), Chapter 12: Animal Diversity of India, p.193; Geography of India, Majid Husain (9th ed.), Contemporary Issues, p.25

8. The Influenza Virus: Strains and Impacts (intermediate)

Influenza, commonly known as the 'flu', is a highly contagious viral infection that primarily attacks the respiratory system. Unlike bacterial infections such as Tuberculosis (caused by Mycobacterium) or Typhoid, influenza is caused by a group of viruses that spread through airborne droplets when an infected person coughs, sneezes, or talks Science, Class VIII, Chapter 3: Health: The Ultimate Treasure, p. 33. The symptoms are distinct and sudden, typically involving high fever, nasal congestion, sore throat, and severe body aches. Because it is viral, prevention through hygiene—such as frequent handwashing and isolation of patients—is often more effective than post-infection treatment Science, Class VIII, Chapter 3: Health: The Ultimate Treasure, p. 33.

For administrative and public health purposes, understanding the strains of influenza is vital. We generally focus on Type A and Type B viruses. Type A is particularly significant because it can jump between species (zoonotic), leading to pandemics. Two major examples that have impacted India are:

• Avian Influenza (Bird Flu): Affects poultry and can occasionally infect humans. India faced its first major outbreak in 2006 in Maharashtra, leading to the establishment of active surveillance and vaccine reserves Geography of India, Majid Husain, Resources, p. 39.
• Swine Flu (H1N1): This strain caused a massive outbreak in 2015, particularly in Rajasthan and Gujarat, resulting in over 2,000 deaths and highlighting the need for robust epidemic management Geography of India, Majid Husain, Contemporary Issues, p. 37.

Feature	Influenza (Flu)	Bacterial Diseases (e.g., TB/Typhoid)
Causal Agent	Virus	Bacteria
Primary Target	Respiratory Tract	Varies (Lungs for TB, Gut for Typhoid)
Prevention	Vaccines/Hygiene	Antibiotics/Vaccines/Sanitation

Sources: Science, Class VIII (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.33; Geography of India, Majid Husain (9th ed.), Resources, p.39; Geography of India, Majid Husain (9th ed.), Contemporary Issues, p.37

9. Solving the Original PYQ (exam-level)

Now that you have mastered the classification of microorganisms, this question allows you to apply that knowledge to real-world health scenarios. In the UPSC Preliminary Examination, distinguishing between viral and bacterial pathogens is a high-yield area. The core concept here is identifying the causative agent for each disease. While many symptoms like fever or cough can overlap, the biological origin of the pathogen dictates the treatment approach and the answer to this question, as emphasized in Science, Class VIII NCERT.

To arrive at the correct answer, we must categorize each option based on its pathogen. Influenza, commonly known as the flu, is the correct choice as it is caused by the influenza virus, which primarily targets the respiratory system. You can recall from your studies that viruses require a host cell to replicate, which is a key characteristic of the flu's rapid spread. Therefore, (C) Influenza is the only viral disease among the choices, a fact corroborated by Environment, Shankar IAS Academy in the context of wildlife and human health.

The other options serve as common traps because they are well-known infectious diseases, but they are all bacterial in nature. Tuberculosis is caused by Mycobacterium tuberculosis, Typhoid by Salmonella Typhi, and Diphtheria by Corynebacterium diphtheriae. UPSC frequently uses these major public health diseases to test if students can accurately distinguish between different classes of pathogens rather than just memorizing symptoms. Understanding these distinctions is crucial for public health policy and disease management topics often found in the Mains syllabus as well.