Which one of the following diseases in humans can spread through air ?

1. Classification of Human Diseases (basic)

Welcome to our first step in understanding human health. To master the study of diseases, we must first learn how to categorize them. At the most fundamental level, human diseases are classified into two broad groups based on whether they can spread from one person to another: Communicable (Infectious) and Non-communicable (Non-infectious) diseases.

Communicable diseases are caused by biological agents called pathogens—such as bacteria, viruses, fungi, or worms—that invade the body. These diseases are 'contagious' or 'infectious' because they can be transmitted through air, water, food, physical contact, or vectors like mosquitoes Science, Class VIII. NCERT (Revised ed 2025), Chapter 3, p.32. On the other hand, Non-communicable diseases (NCDs) are not caused by pathogens and cannot be 'caught' from someone else. Instead, they arise from a combination of genetic factors, environmental influences, and lifestyle choices such as diet and exercise Science, Class VIII. NCERT (Revised ed 2025), Chapter 3, p.35.

While infectious diseases like Malaria and Typhoid have historically been major concerns, modern India is seeing a significant rise in NCDs like diabetes and heart disease due to sedentary lifestyles and processed diets Science, Class VIII. NCERT (Revised ed 2025), Chapter 3, p.32. Understanding this distinction is vital for public health because the prevention strategies for each category are vastly different—ranging from vaccines for infections to lifestyle changes for chronic conditions Science, Class VIII. NCERT (Revised ed 2025), Chapter 3, p.42.

Feature	Communicable Diseases	Non-communicable Diseases
Cause	Pathogens (Bacteria, Viruses, etc.)	Lifestyle, Genetics, Deficiency, or Environment
Spread	Spreads from person to person	Does not spread between individuals
Examples	Dengue, COVID-19, Tuberculosis	Cancer, Diabetes, Goitre, Asthma
Prevention	Vaccines, Sanitation, Hygiene	Healthy diet, Exercise, Avoiding toxins

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.32; Science, Class VIII. NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.35; Science, Class VIII. NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.42

2. Microorganisms: Bacterial and Viral Pathogens (basic)

To understand human infectious diseases, we must first meet the "troublemakers" known as pathogens. These are biological agents—like bacteria, viruses, fungi, or protozoa—that cause disease. In the world of public health, we distinguish between communicable diseases, which are caused by pathogens and can spread from person to person (like the flu or COVID-19), and non-communicable diseases (NCDs), which are linked to lifestyle or genetics (like diabetes or goitre) and do not spread through contact Science, Class VIII (NCERT), Chapter 3, p. 32.

While both bacteria and viruses cause illness, they are fundamentally different biological entities. Bacteria are single-celled, living organisms that can survive on their own in various environments, including soil, water, and the human body. In contrast, viruses are much smaller and are not considered truly "alive" until they enter a living host cell to replicate. This biological difference is why antibiotics are effective against bacteria but have no effect on viral infections like the common cold or flu Science, Class VIII (NCERT), Chapter 3, p. 44.

Pathogens use various "vehicles" to travel from one person to another. These modes of transmission are crucial for UPSC aspirants to categorize:

Mode of Transmission	How it Works	Example Diseases
Airborne	Pathogens travel in tiny droplets when an infected person coughs or sneezes.	Tuberculosis (Bacterial), COVID-19 (Viral)
Vector-borne	Carried by intermediate organisms like mosquitoes or ticks.	Dengue (Viral), Malaria (Protozoan)
Direct Contact	Spread through blood, intimate contact, or from mother to child.	HIV-AIDS (Viral), Hepatitis B (Viral)

Environmental conditions also play a huge role in how these pathogens thrive. For instance, high humidity and shaded environments often favor the growth of bacterial and fungal diseases Environment and Ecology, Majid Hussain, Locational Factors of Economic Activities, p. 27. Similarly, stagnant water provides a breeding ground for mosquitoes, leading to seasonal spikes in viral diseases like Dengue Science, Class VIII (NCERT), Chapter 3, p. 44.

Sources: Science, Class VIII (NCERT), Chapter 3: Health: The Ultimate Treasure, p.32, 35, 44; Environment and Ecology, Majid Hussain, Locational Factors of Economic Activities, p.27

3. Non-Communicable Diseases and Deficiency Disorders (basic)

While infectious diseases are caused by external pathogens, Non-Communicable Diseases (NCDs) and Deficiency Disorders arise from internal factors, lifestyle choices, or the lack of essential nutrients. Unlike the flu or tuberculosis, you cannot "catch" these conditions from another person. They are generally categorized into two groups: those caused by a lack of specific nutrients (deficiency diseases) and those that persist over a long period due to biological or lifestyle factors (chronic diseases).

Deficiency disorders occur when the body does not receive enough of a particular vitamin, mineral, or hormone. For instance, Iodine is vital for the synthesis of thyroxin; a lack of it leads to Goitre, characterized by a swollen neck due to an enlarged thyroid gland Science, Class X, Control and Coordination, p.110. Similarly, a deficiency in Iron or Vitamin B12 often leads to Anaemia, a common blood-related health problem particularly prevalent among adolescent girls in India Science, Class VII, Adolescence: A Stage of Growth and Change, p.80. Even hormonal imbalances, such as a lack of Growth Hormone during childhood, can lead to conditions like dwarfism Science, Class X, Control and Coordination, p.110.

On the other hand, Chronic Diseases like Diabetes, Cancer, and Asthma typically last for more than three months and often result from a complex mix of genetics and lifestyle Science, Class VIII, Health: The Ultimate Treasure, p.36. Diabetes, for example, is increasingly common in India and is linked to hormonal imbalances, lack of physical activity, and unhealthy eating habits. Understanding these differences is crucial: while we use hygiene and vaccines to fight infectious diseases, we use balanced nutrition and lifestyle changes to manage and prevent non-communicable ones.

Type of Disorder	Primary Cause	Examples
Nutritional Deficiency	Lack of vitamins/minerals in diet	Goitre (Iodine), Anaemia (Iron)
Hormonal Imbalance	Over/under secretion by glands	Dwarfism (Growth Hormone), Diabetes (Insulin)
Chronic/Lifestyle	Habits, environment, or genetics	Asthma, Hypertension, Cancer

Sources: Science, Class X, Control and Coordination, p.110; Science, Class VIII, Health: The Ultimate Treasure, p.36; Science, Class VII, Adolescence: A Stage of Growth and Change, p.80; Geography, Class IX, Population, p.53

4. Vector-Borne Diseases and Transmission Dynamics (intermediate)

In the study of infectious diseases, vectors act as the biological bridge between an infected host and a healthy individual. Unlike diseases spread through direct contact or respiratory droplets, Vector-Borne Diseases (VBDs) require a living organism—usually an arthropod like a mosquito, tick, or flea—to carry and transmit the pathogen. For instance, while some diseases are transmitted by direct contact (like sneezing) or contaminated food and water, blood-sucking arthropods are considered far more effective transmitters Geography of India, Majid Husain, Contemporary Issues, p.25. This transmission dynamic is often complex; for example, in the case of Dengue, a mosquito only becomes infectious if it bites an infected human or monkey during the first three days of the victim's illness Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.79.

The prevalence of these diseases is heavily dictated by environmental and climatic conditions. Diseases like Malaria, Dengue, and Chikungunya are inherently seasonal, showing fluctuating trends that peak when conditions are favorable for vector breeding, such as during the rainy season Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.79. Dengue, often colloquially called 'Break-bone Fever' due to the extreme joint pain and stiffness it causes, is primarily spread by the Aedes aegypti (yellow-fever mosquito) and the Aedes albopictus (Asiatic tiger mosquito) Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.79. Malaria, meanwhile, remains a significant health challenge in India, particularly in tropical and sub-tropical regions like Assam, Chhattisgarh, and the Indira Gandhi Canal Command area Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.79.

Disease	Primary Vector	Key Characteristic
Dengue	Aedes aegypti	Severe joint pain ('Break-bone fever')
Malaria	Anopheles mosquito	Periodic chills, fever, and spleen enlargement
Chikungunya	Aedes species	Fever and associated arthritis symptoms

From a policy perspective, the management of VBDs in India is centralized under the National Vectorborne Disease Control Programme (NVBDCP), which operates under the National Rural Health Mission. Because specific vaccines for diseases like Dengue are not always available, the primary strategy focuses on Integrated Vector Control—which includes using insecticides (like pyrethroids), securing window screens, and early case detection to break the cycle of transmission Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.80.

Sources: Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.79; Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.80; Geography of India, Majid Husain, Contemporary Issues, p.25

5. Blood-Borne and Sexually Transmitted Infections (STIs) (intermediate)

Blood-borne and Sexually Transmitted Infections (STIs) represent a category of diseases where the pathogen is transmitted through direct contact with infected bodily fluids. Unlike airborne diseases that travel through the environment, these require an intimate connection or a direct breach of the body's physical barriers. In the context of sexual health, the act involves a close connection of mucous membranes, making it a high-risk route for pathogens like bacteria and viruses to migrate from one person to another Science, Class X, How do Organisms Reproduce?, p.125.

While many STIs are localized, others, like HIV (Human Immunodeficiency Virus), have systemic effects. HIV is a unique viral infection because it specifically targets Helper T cells—a critical component of our immune system's defense mechanism. When these cells are destroyed over time, the body becomes vulnerable to various opportunistic infections and cancers. This advanced stage of the infection is known as AIDS (Acquired Immuno Deficiency Syndrome) Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.80-81. It is important to distinguish between the causative agent (the virus) and the clinical condition (the syndrome).

It is vital to categorize these infections by their biological nature to understand their treatment:

Type of Pathogen	Examples	Common Prevention/Treatment
Bacterial	Gonorrhoea, Syphilis	Can often be treated with antibiotics; condoms reduce transmission.
Viral	Warts, HIV-AIDS, Hepatitis B	Managed with antivirals (no cure for some); vaccines exist for some (e.g., Hep B).

Beyond sexual contact, these pathogens can also be transmitted through vertical transmission—from mother to child. This can occur across the placenta (which normally serves as a site for oxygen and glucose exchange), during the birth process, or through breastfeeding Science, Class X, How do Organisms Reproduce?, p.124. Understanding these diverse routes—blood, semen, and breast milk—is essential for public health strategies and personal safety.

Sources: Science, Class X (NCERT 2025 ed.), How do Organisms Reproduce?, p.124-125; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Natural Hazards and Disaster Management, p.80-81

6. Respiratory Infections and Airborne Transmission (exam-level)

To understand respiratory infections, we must first look at the physics of how we breathe and interact. When an infected person coughs, sneezes, or even speaks, they release a spray of moisture. These particles vary in size: larger droplets settle quickly due to gravity, but much smaller particles, often called aerosols or droplet nuclei, can remain suspended in the air for hours and travel significant distances on air currents. This is the essence of airborne transmission.

Tuberculosis (TB), caused by the bacterium Mycobacterium tuberculosis, is the classic example of an airborne disease. Because the bacteria are so resilient, they can linger in the air of a room long after an infected person has left. Once inhaled, these bacteria primarily target the lungs, leading to symptoms like a persistent cough, chest pain, fever, and night sweats Science, Class VIII NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.34. This distinguishes it from diseases like Cholera or Typhoid, which are water-borne and spread through the ingestion of contaminated water or food rather than through the air Science, Class VIII NCERT (Revised ed 2025), Chapter 3, p.44.

Understanding the mode of transmission is critical for public health. For airborne diseases, the primary line of defense involves respiratory hygiene (covering the mouth and nose), ensuring proper ventilation in indoor spaces to disperse aerosols, and vaccination (such as the BCG vaccine for TB). Unlike vector-borne diseases like Dengue, which require a mosquito to move the pathogen from person to person, airborne infections move directly through the shared atmosphere we breathe Geography of India, Majid Husain, Contemporary Issues, p.25.

Sources: Science, Class VIII NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.34; Science, Class VIII NCERT (Revised ed 2025), Chapter 3: Health: The Ultimate Treasure, p.44; Geography of India, Majid Husain (McGrawHill 9th ed.), Contemporary Issues, p.25

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental classification of diseases, you can see how this question tests your ability to categorize health conditions by their mode of transmission. The core concept here is identifying infectious agents and the specific medium they use to travel from a reservoir to a susceptible host. In your recent modules, we explored how respiratory pathogens utilize "droplet nuclei" to stay suspended in the atmosphere; this is the defining characteristic of airborne diseases, which this question targets directly.

To arrive at the correct answer, you must walk through the pathway of infection for each option. Tuberculosis (TB) is the clear fit because it is caused by the bacterium Mycobacterium tuberculosis, which resides in the lungs and is expelled into the environment through coughing or sneezing. As explained in Science, Class X (NCERT 2025 ed.), these causative agents are released in small droplets that can travel long distances. Therefore, the logical choice is (B) Tuberculosis, as it is the only pathogen listed that utilizes the atmosphere as its primary transport route.

A common UPSC strategy is to mix different types of health conditions to test your conceptual boundaries. For example, Dengue is a frequent trap; while it is widespread, it is a vector-borne disease requiring a mosquito to spread, not air. Similarly, HIV-AIDS is a viral infection that requires intimate contact or blood exchange, as noted in Science, Class VIII NCERT. Finally, Goitre is a non-communicable deficiency caused by a lack of iodine, meaning it cannot "spread" at all. By distinguishing between vectors, direct contact, and nutritional gaps, you can easily eliminate the distractors and focus on the respiratory transmission of TB.