Pathogens, Common Diseases and Their Prevention: Quick Revision

A brief note on Pathogens, Common Diseases and Their Prevention. Read it before a test. Try each question at the end before you open its answer.

The diseases

• Typhoid: Salmonella typhi, a bacterium. Food and water. Small intestine, then blood. High fever. Widal test. Carriers can spread it.
• Pneumonia: Streptococcus pneumoniae and Haemophilus influenzae, bacteria. The alveoli fill with fluid. Droplets and shared utensils.
• Common cold: rhinoviruses. Nose and respiratory passage, not the lungs. Droplets and soiled objects.
• Dengue and chikungunya: viruses carried by the female Aedes, which breeds in clean standing water.
• Malaria: Plasmodium, a protozoan, carried by the female Anopheles.
• Amoebiasis: Entamoeba histolytica, a protozoan of the large intestine. Houseflies are mechanical carriers.
• Ascariasis: Ascaris, a roundworm. Its eggs leave in the faeces and contaminate soil, water and food.
• Filariasis: Wuchereria, in the lymphatic vessels of the lower limbs. A female mosquito carries it. Elephantiasis.
• Ringworm: fungi (Microsporum, Trichophyton, Epidermophyton). Dry, scaly patches and itching. Heat and moisture help them.

Malaria in brief

• Sporozoites enter the human, multiply in the liver cells and then in the red blood cells.
• The red cells burst and release haemozoin: chill and high fever. Tertian fever comes every 48 hours, and quartan fever every 72 hours.
• Gametocytes form in red cells. The female Anopheles sucks them up. The gametes fuse in her gut.
• Sporozoites are stored in her salivary glands and are injected at the next bite.

Prevention

• Personal hygiene: clean body, clean water and food.
• Public hygiene: waste disposal, clean water tanks.
• Control of vectors: no standing water, nets, Gambusia, insecticide, wire mesh.
• Vaccination.

A worked example

A culture has 20 bacteria and doubles every 30 minutes. How many are there after 2 hours?

1. The doublings are $\dfrac{120}{30} = 4$.
2. The number is $20 \times 2^{4} = 320$.

Practice with new numbers

A program to try

The program counts the diseases of the chapter by the kind of pathogen.

Mistakes to avoid

• Saying that ringworm is caused by a worm. Fungi cause it.
• Saying that a cold infects the lungs. It stays in the nose and the respiratory passage.
• Mixing up the two infective stages of the parasite. Sporozoites go to the human. Gametocytes go to the mosquito.
• Putting the fusion of the gametes in the human. It happens in the gut of the mosquito.

Check yourself

1. Which mosquito spreads malaria?

2. What do the filarial worms infect?

3. What is released when the red cells burst in malaria?

4. Name one measure of vector control.

Sources

• World Health Organization (2024). Fact sheets: Malaria; Dengue and severe dengue; Chikungunya; Typhoid.
• NCERT (2023). Biology, Textbook for Class XII, chapter 7.