Unit 11 Infectious diseases, immunity and vaccination
Key unit competence: Explain how infectious diseases spread, how the body defends itself and how vaccination protects against disease.
Key ideas
- An infectious (communicable) disease is caused by a pathogen (bacterium, virus, fungus, protoctist or worm) and can spread from one person to another. The organism that carries a pathogen between hosts without suffering from it is a vector (e.g. the Anopheles mosquito).
- Ways of transmission: air droplets (TB, influenza, measles, COVID-19), contaminated food and water (cholera, typhoid, dysentery), vectors (malaria by female Anopheles mosquito, sleeping sickness by tsetse fly), direct contact and body fluids (HIV, Ebola, STIs, ringworm), from mother to child.
- Common diseases: malaria (Plasmodium; fever, shivering, headache; prevented by mosquito nets, repellents, draining stagnant water, spraying; treated with antimalarials); cholera (bacterium; severe watery diarrhoea; prevented by clean water, latrines, hand washing; treated with ORS and antibiotics); typhoid (Salmonella typhi); tuberculosis; measles (virus, rash, prevented by vaccine).
- First line of defence (stops pathogens entering): skin (barrier), mucus and cilia in airways, stomach acid, tears and saliva with lysozyme, blood clotting that seals wounds.
- Second line: phagocytes (white blood cells) engulf and digest pathogens by phagocytosis; inflammation brings more blood and white cells to an infected area.
- Third line (specific immune response): lymphocytes recognise antigens (molecules on the surface of pathogens) and produce antibodies with a shape that fits a particular antigen. Antibodies clump pathogens together (agglutination), neutralise toxins and mark pathogens for phagocytes.
- Primary response: on first infection, antibodies are made slowly and the person may become ill. Memory cells remain. Secondary response: on a second infection, many antibodies are made quickly, so the person usually does not become ill: they are immune.
- Active immunity: the body makes its own antibodies, either naturally (after infection) or artificially (by vaccination); it lasts long. Passive immunity: ready-made antibodies are received, naturally (from mother through the placenta and breast milk) or artificially (antibody serum, e.g. anti-venom); it is short-lived because no memory cells are made.
- A vaccine contains dead or weakened pathogens, or harmless parts (antigens) or toxoids. It stimulates a primary response and memory cells without causing the disease. Rwanda's programme includes BCG, polio, measles-rubella, pentavalent, pneumococcal, rotavirus and HPV vaccines. Herd immunity: when most people are vaccinated, the pathogen cannot spread easily, protecting those who cannot be vaccinated.
- Autoimmunity: the immune system attacks the body's own cells. Example: in type 1 diabetes, it destroys the insulin-producing cells of the pancreas. Allergy is an over-reaction to a harmless substance (pollen, dust).
Common misconceptions
- "Antibiotics cure viral diseases such as influenza." No: antibiotics kill bacteria only; they do not affect viruses.
- "A vaccine cures a disease." No: a vaccine prevents disease by preparing the immune system before infection.
- "Mosquitoes cause malaria." No: malaria is caused by the protoctist Plasmodium; the mosquito is the vector.
- "Antibodies from breast milk give a baby lifelong immunity." No: this passive immunity lasts only weeks or months.
Worked examples
Example 1. Explain why a person who had measles as a child does not get it again.
Solution
During the first infection, lymphocytes made antibodies against measles antigens and formed memory cells. On a new infection, the memory cells quickly make large amounts of antibodies (secondary response), destroying the virus before symptoms develop. The person has natural active immunity.
Example 2. Explain how draining stagnant water and sleeping under treated nets reduce malaria.
Solution
Female Anopheles mosquitoes lay eggs in stagnant water, where larvae develop; draining it reduces the number of mosquitoes. Treated nets stop mosquitoes biting at night and kill them, so Plasmodium cannot be passed from infected to healthy people.
Practice questions
Define: (a) pathogen; (b) vector; (c) antigen; (d) antibody.
Show answer
(a) An organism that causes disease. (b) An organism that carries a pathogen from one host to another. (c) A molecule (on a pathogen) that triggers antibody production. (d) A protein made by lymphocytes that binds a specific antigen.
Common mistake: Mixing up antigen and antibody.
State the ways in which infectious diseases are transmitted, with an example of each.
Show answer
Air droplets (TB, flu); contaminated food and water (cholera, typhoid); vectors (malaria by mosquito); body fluids and contact (HIV, Ebola); mother to child (HIV).
Common mistake: Saying malaria spreads through water.
Describe the first line of defence of the body.
Show answer
Skin acts as a barrier; mucus traps and cilia sweep pathogens out of airways; stomach acid kills swallowed pathogens; tears and saliva contain enzymes; blood clots seal wounds.
Common mistake: Including antibodies in the first line.
Distinguish between active and passive immunity.
Show answer
Active: the body makes its own antibodies (after infection or vaccination); memory cells form; long-lasting. Passive: ready-made antibodies are received (from mother or serum); no memory cells; short-lived.
Common mistake: Saying vaccines give passive immunity.
Explain how a vaccine protects a person.
Show answer
It contains harmless antigens; lymphocytes make antibodies and memory cells (primary response). If the real pathogen enters later, memory cells quickly make many antibodies (secondary response) and destroy it before illness.
Common mistake: Saying the vaccine kills pathogens directly.
State three ways to prevent and control malaria.
Show answer
Sleep under insecticide-treated nets; drain or cover stagnant water; spray houses with insecticide; use repellents; clear bushes; early diagnosis and treatment.
Common mistake: Saying malaria spreads by touching a patient.
State the signs, cause and prevention of cholera.
Show answer
Signs: severe watery diarrhoea, vomiting, dehydration. Cause: the bacterium Vibrio cholerae. Prevention: safe drinking water, proper latrines, hand washing, food hygiene.
Common mistake: Treating cholera only with antibiotics and no fluids.
Explain what is meant by autoimmunity, with one example.
Show answer
The immune system attacks the body's own cells as if they were foreign; e.g. in type 1 diabetes it destroys the insulin-producing cells of the pancreas.
Common mistake: Confusing autoimmunity with immunity.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Antibiotics are used to treat infections caused by: a. dead or weakened pathogens 2. A vaccine contains: b. bacteria 3. The BCG vaccine protects against: c. acting as a barrier to pathogens 4. The skin protects the body by: d. tuberculosis Show answer
1 → b (bacteria) · 2 → a (dead or weakened pathogens) · 3 → d (tuberculosis) · 4 → c (acting as a barrier to pathogens)
- They do not affect viruses.
- It triggers an immune response safely.
- It is given at birth.
- It is part of the first line of defence.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
primary response · secondary response · phagocytes · Plasmodium, a protoctist- Malaria is caused by .
- White blood cells that engulf pathogens are .
- The faster, larger antibody response to a second infection is the .
Show answer
1: Plasmodium, a protoctist · 2: phagocytes · 3: secondary response
- The mosquito transmits it.
- This process is phagocytosis.
- Memory cells make it possible.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Molecules on the surface of a pathogen that trigger antibody production are: a. cells that make insulin 2. Stomach acid helps defence by: b. killing pathogens swallowed with food 3. In type 1 diabetes, the immune system attacks: c. vector 4. An organism that carries a pathogen from one host to another is a: d. antigens Show answer
1 → d (antigens) · 2 → b (killing pathogens swallowed with food) · 3 → a (cells that make insulin) · 4 → c (vector)
- Each antibody fits a specific antigen.
- It is part of the first line of defence.
- This is an autoimmune disease.
- The Anopheles mosquito is a vector.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
lymphocytes · artificial active immunity · natural passive immunity · natural active immunity- Antibodies are produced by .
- Antibodies passed from mother to baby in breast milk give .
- A child gets measles once and is protected for life. This is .
Show answer
1: lymphocytes · 2: natural passive immunity · 3: natural active immunity
- Lymphocytes are white blood cells.
- The baby does not make the antibodies.
- The child's own body made the antibodies after infection.