Diseases: Cause and Control — ICSE Class 9 Biology Important Questions

Answer: (c) Diabetes.

Diabetes is a metabolic, non-communicable disease that does not spread from person to person. Cholera, tuberculosis and malaria are all communicable diseases caused by pathogens.

Answer: (b) Female $Anopheles$Anopheles mosquito.

Malaria is caused by the protozoan $Plasmodium$Plasmodium, which is transmitted by the bite of the female $Anopheles$Anopheles mosquito; only the female feeds on blood.

Answer: (c) Fungi.

Ringworm (of the skin, scalp) and athlete's foot are fungal infections, spread by contact with infected persons, animals or shared articles like towels and combs.

Answer: (b) Faecal-oral (contaminated food/water).

Typhoid (caused by $Salmonella\ typhi$Salmonella typhi) spreads when food or water contaminated with the faeces of a patient or carrier is consumed; houseflies and unwashed hands help carry the germ to food.

Answer: (a) Non-communicable diseases (e.g. scurvy, diabetes, cancer) are not caused by pathogens but by nutritional, metabolic, lifestyle or genetic causes, so they cannot be transmitted; R correctly explains A.

(i) Pathogen: A disease-causing organism (germ) such as a bacterium, virus, protozoan, fungus or worm that lives in or on a host and produces disease.

(ii) Vector: An organism (usually an insect) that carries a pathogen from an infected host to a healthy host without itself suffering from the disease, e.g. the female $Anopheles$Anopheles mosquito for malaria.

(i) Tuberculosis: Caused by the bacterium $Mycobacterium\ tuberculosis$Mycobacterium tuberculosis; transmitted through air by droplets released when a patient coughs or sneezes.

(ii) Cholera: Caused by the bacterium $Vibrio\ cholerae$Vibrio cholerae; transmitted through water and food contaminated with the patient's faeces.

Communicable diseases

• Caused by pathogens (bacteria, viruses, protozoa, etc.).
• Spread from an infected person to a healthy person by air, water, food, contact or vectors.
• Can occur as epidemics.
• Examples: tuberculosis, malaria, cholera.

Non-communicable diseases

• Not caused by pathogens; due to deficiency, lifestyle, degeneration or genetic defects.
• Do not spread from person to person.
• Usually confined to the affected individual.
• Examples: diabetes, scurvy, cancer.

Thus communicable diseases are infectious and transmissible, whereas non-communicable diseases are non-infectious.

Communicable diseases spread mainly in the following ways:

1. Air-borne (droplet) transmission: Pathogens in tiny droplets are released when a patient coughs, sneezes or talks and are inhaled by others, e.g. tuberculosis, common cold, influenza.

2. Water- and food-borne transmission: Drinking water or food contaminated with pathogens, e.g. cholera, typhoid.

3. Contact transmission: By direct touch or by using articles of an infected person (towels, combs), e.g. ringworm, scabies.

4. Vector-borne transmission: Through insects that carry the pathogen, e.g. malaria (mosquito), plague (rat flea).

Knowing the mode helps in breaking the chain and controlling the disease.

Malaria is spread by the female $Anopheles$Anopheles mosquito, which breeds in stagnant water and bites at night.

Draining stagnant water: The mosquito lays eggs and its larvae develop in still water. Removing stagnant water destroys the breeding grounds, so larvae cannot develop into adults.

Spraying oil (or kerosene) on ponds: A thin oil film spreads over the water surface and cuts off the air supply to the larvae and pupae, which breathe at the surface; they suffocate and die.

Using mosquito nets: Since $Anopheles$Anopheles bites at night, sleeping under nets prevents the adult mosquito from biting and transmitting $Plasmodium$Plasmodium.

Together the first two attack the aquatic (larval/pupal) stages and the last prevents the adult from infecting people, breaking the cycle at more than one point.

1. Bacteria — Tuberculosis (caused by $Mycobacterium\ tuberculosis$Mycobacterium tuberculosis). Symptoms: persistent cough, sometimes with blood, fever, weight loss and weakness.

2. Viruses — Influenza/common cold. Symptoms: running nose, sneezing, sore throat, mild fever and body ache.

3. Protozoa — Malaria (caused by $Plasmodium$Plasmodium). Symptoms: recurring high fever with chills and shivering, headache and sweating.

4. Fungi — Ringworm. Symptoms: ring-shaped red, itchy patches on the skin or scalp.

5. Helminths (worms) — Ascariasis (roundworm) or tapeworm infection. Symptoms: abdominal pain, indigestion, weakness and anaemia due to loss of nutrients.

These five groups — bacteria, viruses, protozoa, fungi and worms — cover the main categories of disease-causing organisms in humans.

Communicable diseases can be prevented and controlled by attacking the pathogen, its route of spread and the host:

1. Maintaining personal hygiene: Washing hands before eating and after using the toilet, bathing, and keeping nails and clothes clean reduce contact with germs.

2. Ensuring safe food and water: Drinking boiled or filtered water, covering food, and eating freshly cooked food prevent water- and food-borne diseases like cholera and typhoid.

3. Sanitation and cleanliness: Proper disposal of sewage and refuse and keeping surroundings clean prevent breeding of flies and mosquitoes.

4. Vector control: Draining stagnant water, spraying insecticides and using mosquito nets control vector-borne diseases such as malaria and dengue.

5. Isolation of patients: Keeping infected persons separate prevents spread to healthy people.

6. Vaccination (immunisation): Vaccines protect against diseases such as tuberculosis, measles and polio.

7. Health education: Making people aware of causes and prevention encourages good habits.

Together these break the chain of infection and keep the community disease-free.

(i) The disease is malaria, caused by the protozoan $Plasmodium$Plasmodium.

(ii) The vector is the female $Anopheles$Anopheles mosquito. It breeds in clean, stagnant (still) water such as that collected in discarded tyres, open drums, coolers and puddles.

(iii) During the rainy season rainwater collects in many containers and low-lying areas, giving the mosquito abundant stagnant water to breed in; the mosquito population rises sharply, so more people are bitten and infected.

(iv) Two measures: (1) Empty or cover all containers holding stagnant water and drain the puddles so mosquitoes cannot breed. (2) Use mosquito nets/repellents and spray insecticides to prevent bites and kill adult mosquitoes.