Q.What do you understand by the following: Infectious diseases.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Disease Transmission Modes
Let’s begin with something you already know. When someone in your family catches a cold, you instinctively avoid sharing their glass or sitting too close. That instinct is your brain recognising that something — a disease — can travel from one person to another. The how of that travel is what we call disease transmission modes.
In everyday language, a transmission mode is simply the route a germ takes to leave an infected person and reach a healthy one. Think of it as the germ’s travel plan. If the germ cannot travel, it cannot cause new infections. So understanding these routes is the first step in stopping outbreaks — whether it’s a common flu or a more serious epidemic.
The two big families: Direct and Indirect transmission
The NCERT textbook (Class XII, Biology, Chapter 8 — Human Health and Disease) divides transmission into two main categories. This is the standard classification you need to remember.
1. Direct Transmission — The germ moves straight from the infected person to the healthy person, with no intermediate object or living being involved. This can happen through:
- Physical contact: Touching, shaking hands, kissing, or sexual contact. Diseases like the common cold, scabies, and sexually transmitted infections (e.g., syphilis, HIV) spread this way.
- Droplet infection: When an infected person coughs, sneezes, or even talks, tiny droplets of saliva or mucus fly out. If you are within about a metre, you can inhale them. This is how influenza, the common cold, and COVID-19 spread.
- Direct exposure to infected tissue or blood: For example, a needle-stick injury in a hospital, or a mother passing an infection to her baby during pregnancy (congenital transmission).
Direct transmission is often the easiest to interrupt. Simple actions — like maintaining distance, wearing a mask, or washing hands — break the chain because the germ has no vehicle to ride on.
2. Indirect Transmission — Here, the germ needs a middleman to reach a new host. The middleman can be:
- A vehicle: This is a non-living thing that carries the germ. Common vehicles include:
- Air: Dust particles or droplet nuclei (the dried remnants of droplets) can float for hours. Measles and tuberculosis spread through air.
- Water: Contaminated drinking water spreads cholera, typhoid, and hepatitis A.
- Food: Improperly cooked or stored food can carry salmonella or E. coli.
- Fomites: Inanimate objects like door handles, towels, syringes, or surgical instruments. Hepatitis B can spread through contaminated needles.
- A vector: This is a living organism — usually an insect or animal — that carries the germ from one host to another. The vector itself is not infected (or is only incidentally infected). Classic examples:
- Mosquitoes: Female Anopheles mosquito transmits malaria; Aedes mosquito transmits dengue and chikungunya.
- Fleas: Rat fleas transmit the plague bacterium.
- Ticks: Spread Lyme disease.
The key difference between a vehicle and a vector is that a vehicle is non-living, while a vector is a living creature. Both are intermediate agents — the germ does not jump directly from person to person.
Why this matters beyond biology
For a commerce or humanities student, the importance of transmission modes is not just medical — it is social and economic. Think about it:
- If a disease spreads through water (like cholera), the solution is not a vaccine alone — it is clean water supply, sanitation infrastructure, and public awareness campaigns. That is a government policy and budget issue.
- If a disease spreads through vectors (like malaria), the solution involves mosquito nets, insecticide spraying, and urban planning to eliminate stagnant water. That is a logistics and community management problem.
- If a disease spreads through direct contact (like HIV), the solution involves education, behaviour change, and stigma reduction — which is a social science challenge.
In short, the mode of transmission determines the type of intervention needed. That is why public health officials always ask: “How is this disease spreading?” before they decide what to do.
A quick summary for revision …
Infectious (communicable) diseases are diseases caused by pathogens (bacteria, viruses, fungi, protozoa, helminths) that spread from an infected person to a healthy person through air, water, food, contact or vectors — e.g. tuberculosis, malaria, common cold, typhoid. …
Infectious diseases are caused by pathogens and spread from person to person; e.g. TB, malaria, typhoid.
Concept. Infectious diseases (also called communicable diseases) are diseases that are caused by pathogens (disease-causing micro-organisms) and can be transmitted from an infected individual to a healthy individual. This transmissibility is what distinguishes them from non-infectious diseases (like cancer or diabetes), which are not caused by pathogens and do not spread.
Causative agents: bacteria (e.g. Salmonella → typhoid, Mycobacterium → TB), viruses (common cold, dengue), protozoa (Plasmodium → malaria, Entamoeba → amoebiasis), fungi (ringworm) and helminths (Ascaris, Wuchereria → filariasis).
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Showing the 12 most recent of 14 on this concept.
- CBSE 2026Set A1 markMCQQ.Which of the following diseases is/are transmitted by insect vectors?(a) Typhoid(b) Malaria(c) Filaria(d) Both (B) and (C)
›Reveal solutionSolution
Malaria and filariasis are insect (mosquito) transmitted; typhoid is spread by contaminated food and water.
Malaria is transmitted by the female Anopheles mosquito, and filariasis (elephantiasis) is transmitted by the female Culex mosquito — both are insect vectors. Typhoid, caused by Salmonella typhi, is transmitted through food and water contaminated with …
- CBSE 2025Set ANNUAL1 markQ.Name two diseases whose spread can be controlled by the eradication of Aedes mosquito.
›Reveal solutionSolution
The Aedes mosquito is the vector for several viral diseases, notably dengue and chikungunya, so controlling its population directly limits their spread.
The Aedes mosquito acts as the vector transmitting several important viral diseases to humans, including dengue fever and chikungunya (it can also transmit Zika virus and yellow fever). Since these viral diseases spread only through the bite of an infected Aedes mosquito, eliminating its breeding sites (stagnant/clean standing water) and controlling its population is an …
- CBSE 2024Set 57/2/11 markMCQQ.The vector for dengue fever is : (A) Female Aedes mosquito (B) Female Anopheles mosquito (C) Male Aedes mosquito (D) Female Culex mosquito
›Reveal solutionSolution
Dengue fever is transmitted by the bite of an infected female Aedes mosquito (primarily Aedes aegypti), which requires a blood meal for egg development. The answer is (A).
Why vectors matter in disease transmission
A vector is an organism that transmits a pathogen from one host to another without suffering from the disease itself. For many viral diseases, mosquitoes serve as vectors because the pathogen replicates in the mosquito's body and is passed on when the mosquito feeds on blood.
The key biological detail: only female mosquitoes bite and feed on blood. Males feed on plant nectar and do not require blood meals. Females need the protein from blood to develop their eggs, which is why they are the disease vectors.
Identifying the dengue vector
Different mosquito genera transmit different diseases, and this specificity is crucial for public health control measures.
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Aedes mosquitoes are the primary vectors for dengue fever. The species Aedes aegypti (and to a lesser extent Aedes albopictus) transmit the dengue virus when an infected female bites a human. These mosquitoes are recognizable by their distinctive black-and-white striped legs and body, and they typically bite during the day, especially in the early morning and before dusk.
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Anopheles mosquitoes transmit malaria, not dengue. The Plasmodium parasite that causes malaria requires Anopheles as its vector. …
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- CBSE 2024Set GG1 markMCQQ.The vector of Dengue (Break bone) fever is:(a) Anopheles(b) Culex(c) Aedes(d) None of these
›Reveal solutionSolution
Dengue is spread by the Aedes mosquito, so the answer is (c).
Concept. Many diseases have a specific insect vector. Matching the pathogen to its vector is important: malaria — Anopheles; filariasis — Culex; dengue and chikungunya — Aedes.
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- CBSE 2023Set TERM21 markMCQQ.A disease which can easily transmit from one person to another is called(a) Non-infectious disease(b) Infectious disease(c) Viral disease(d) Bacterial disease
›Reveal solutionSolution
A disease that spreads from an infected person to a healthy person is called an infectious (communicable) disease.
Diseases are broadly grouped into non-infectious (e.g. cancer, allergies) and infectious diseases. Infectious diseases are caused by pathogens such as bacteria, viruses, fungi, protozoans and helminths, and these pathogens can be transmitted from an infected host to a healthy person directly (contact, air, water) or through vectors, which is why such diseases are also called …
- CBSE 2023Set ANNUAL1 markMCQQ.What is a droplet infection?(a) syphilis(b) tetanus(c) typhoid(d) pneumonia
›Reveal solutionSolution
Pneumonia is a droplet infection.
A droplet infection spreads when an infected person coughs or sneezes, releasing tiny droplets carrying the pathogen that a healthy person then inhales. Pneumonia (caused by Streptococcus pneumoniae or Haemophilus influenzae) spreads this way. In contrast, syphilis is a sexually transmitted disease, teta …
- CBSE 2022Set ANNUAL1 markMCQQ.The Pathogen of elephantiasis is -(a) Amoeba(b) Ascaris(c) Filarial worm(d) Plasmodium
›Reveal solutionSolution
Elephantiasis is lymphatic filariasis, caused by filarial worms (Wuchereria bancrofti / W. malayi).
Elephantiasis (filariasis) is a chronic disease caused by filarial worms, chiefly Wuchereria bancrofti and W. malayi. These thread-like helminths live in the lymphatic vessels, and are transmitted by the bite of female Culex mosquitoes. Chronic inflammation and blockage of lymph vessels lead to gross swelling of the affected par …
- CBSE 2022Set ANNUAL1 markMCQQ.Which fever could be confirmed by Widal test ?(a) Malaria(b) Dengue(c) Corona(d) Typhoid
›Reveal solutionSolution
Widal is an antibody-agglutination test that confirms typhoid fever.
Typhoid fever is caused by the bacterium Salmonella typhi. The Widal test is a serological test that detects agglutinating antibodies (against the O and H antigens of Salmonella) in the patient's serum; a rising titre confirms typhoid.
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- CBSE 2021Set D1 markMCQQ.Tuberculosis is transmitted by(a) Water(b) Air(c) Insect(d) Contact
›Reveal solutionSolution
Tuberculosis spreads through air (droplet infection).
Tuberculosis is caused by the bacterium Mycobacterium tuberculosis, which chiefly affects the lungs. When an infected person coughs, sneezes or spits, the bacteria are released in tiny droplets into the air and are inhaled by healthy pe …
- CBSE 2021Set ANNUAL1 markMCQQ.The word pathogen refers to –(i) healthy living organism(ii) cultured micro-organism(iii) inoculated organism(iv) disease causing organism
›Reveal solutionSolution
A pathogen is any organism that causes disease in its host.
The word pathogen comes from Greek (pathos = suffering, genes = producer) and refers to any micro-organism — bacterium, virus, fungus, protozoan, or helminth — capable of causing disease in a host organism, e.g. Salmonella typhi (typhoid), Plasmodium (malaria). It is unrelated to wh …
- CBSE 2019Set ANNUAL1 markQ.Hepatitis is a ____ disease.
›Reveal solutionSolution
Hepatitis means inflammation of the liver, and is most commonly caused by infection with one of the hepatitis viruses (A-E), which differ in their mode of transmission and severity.
- Hepatitis A and E viruses spread mainly through the faecal-oral route via contaminated food or water, and usually cause acute, self-limiting illness.
- Hepatitis B, C, and D viruses spread through blood, body fluids, and from mother to child, and can cause chronic infection leading to liver damage (cirrhosis) or liver cancer over time. …
- CBSE 2019Set ANNUAL1 markMCQQ.Tuberculosis is transmitted by-(a) Air(b) Water(c) Insect(d) Contact
›Reveal solutionSolution
Tuberculosis is transmitted through the air by droplet infection.
Tuberculosis is caused by the bacterium Mycobacterium tuberculosis. When an infected person coughs, sneezes or spits, the pathogen is released in tiny droplets that are inhaled by others, ma …
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