Science notes, chapter 4
Human health
Textbook Science Form 2 (DLP), pages 74–92
What this chapter covers
- Differentiate infectious and non-infectious diseases, and name examples of each (4.1.1)
- Explain how infectious diseases are spread through air, water, contact and vectors (4.1.2)
- Separate the cause of a disease (pathogen) from the way it spreads (medium or vector) (4.1.3)
- Suggest the primary, secondary and tertiary stages that prevent the spread of infectious diseases (4.1.4)
- Explain the first, second and third lines of defence and define antigen, antibody and immunity (4.2.1, 4.2.2)
- Justify the importance of immunisation and read the vaccination schedule (4.2.3)
- Differentiate passive and active immunity, natural and artificial, from antibody graphs (4.2.4)
- Justify practices that strengthen the immune system and explain why individual health matters to the nation (4.2.5, 4.2.6)
4.1Infectious and non-infectious diseases
A disease is an abnormal condition of the body or mind that causes discomfort, difficulty in functioning or stress. A pathogen is an organism that causes disease. A vector is an animal that carries pathogens from one host to another. The host is the person who is infected.
| Infectious disease | Non-infectious disease | |
|---|---|---|
| Cause | Infection by pathogens, directly through a medium or through a vector | Genetic factors or lifestyle |
| Transmission | Can be passed from one person to another | Cannot be passed from one person to another |
| Examples | Tuberculosis, flu, ringworm, tinea, leptospirosis, dengue, malaria, Zika | Cancer, hypertension, diabetes, asthma, cardiovascular diseases |
Pathogens are all viruses and some bacteria, protozoa, fungi and worms. Not all microorganisms are harmful: some bacteria in the large intestine make vitamin K and vitamin B12, which the body absorbs.
"Asthma is caused by a germ" is wrong. Asthma, diabetes and cancer are non-infectious, so you cannot catch them from another person, even by close contact.
How infectious diseases spread
| Medium | How it happens | Examples | Prevention |
|---|---|---|---|
| Air | Droplets from coughs and sneezes, or dust that carries pathogens | Tuberculosis, flu, SARS, H1N1, chicken pox | Cover the mouth and nose when coughing or sneezing; do not spit; avoid crowds; let sunlight (ultraviolet rays) into the house |
| Water | Faecal pathogens pollute drinking water, especially with poor sanitation or after floods | Cholera, typhoid, amoebic dysentery | Chlorinate water supplies and pools; build toilets with good sanitation; boil drinking water; wash hands with soap after using the toilet |
| Contact | Touching infected skin or using an infected person's clothes; body fluids and blood | Ringworm and tinea (fungi); syphilis, gonorrhoea, HIV/AIDS | Do not share personal items; do not share syringes |
| Vector | An animal carries the pathogen to a new host | Dengue, Zika, malaria, typhoid, leptospirosis | Destroy breeding places; fogging; mosquito nets and coils |
Vectors and their pathogens
| Disease | Pathogen | Vector | Way of infection | Symptoms |
|---|---|---|---|---|
| Malaria | Plasmodium malariae | Female Anopheles mosquito | Mosquito bite | Shivering, fever, sweating |
| Dengue | Virus | Aedes mosquito | Mosquito bite | Joint pain, fever, headache, watery eyes |
| Zika | Virus | Aedes mosquito | Mosquito bite | Fever, rashes, joint pain, conjunctivitis |
| Cholera | Vibrio cholerae bacteria | Fly | Contaminated food and water | Diarrhoea, vomiting |
| Typhoid | Salmonella typhi bacteria | Cockroach, fly | Contaminated food and water | Fever, intestinal bleeding, red rashes |
| Leptospirosis | Leptospira sp. bacteria | Rat | Contaminated soil, food and water | Fever, headache, muscle pain |
Tables 4.1 and 4.2: textbook pp. 79–80 (rearranged).
Keep pathogen (the cause) and vector (the carrier) apart. The Aedes mosquito is the vector of dengue; the dengue virus is the pathogen. A mosquito is never the pathogen.
Figure 4.3: Science Form 2 (DLP) textbook, p. 80.
Preventing the spread: three stages
| Stage | What is done |
|---|---|
| Primary | Improve health and hygiene and sanitation; strengthen the body's defence (vaccines and immunisation); health check-ups; a healthy lifestyle (clean air, balanced diet) |
| Secondary | Find cases early (active and passive case detection); treat patients early; separate patients from others |
| Tertiary | Control vector numbers (destroy breeding and hiding places, fogging, fines for dirty food premises); protect hosts (mosquito nets, coils, thick clothes) |
Name the pathogen and the vector of dengue, and state one way to break the spread.
Show answerHide answer
Pathogen: dengue virus. Vector: Aedes mosquito. Break the spread by destroying the places where mosquitoes breed (stagnant water), or by fogging, or by using mosquito nets.
4.2Body defence
Pathogens enter the body through the respiratory system, the digestive system, the excretory system and the skin. The body has three lines of defence.
| Line of defence | Type | What it does |
|---|---|---|
| First | Non-specific | Skin: a tough layer; sweat and sebum contain chemicals that kill microorganisms. Mucous membrane: lines the digestive and respiratory tracts; nasal hairs and mucus trap microorganisms; earwax and tears act as antiseptics. Prevents pathogens from entering. |
| Second | Non-specific | White blood cells in the bloodstream engulf and digest pathogens with enzymes (phagocytosis). |
| Third | Specific | White blood cells make antibodies against the particular pathogen. Antibodies attach to pathogens and make them clump together. |
Figure 4.4: textbook p. 82.
Antigen — a foreign substance that induces the production of antibodies (pathogens, toxin molecules, blood cells from another blood group).
Antibody — a protein made by white blood cells into the bloodstream in response to an antigen.
Immunity — the ability of the body to resist pathogens before it is infected.
Antibody — a protein made by white blood cells into the bloodstream in response to an antigen.
Immunity — the ability of the body to resist pathogens before it is infected.
Skin, mucous membrane and white blood cells defend against any pathogen (non-specific). Only antibodies are specific: each antibody fits one antigen.
Tears, earwax and nasal hairs belong to which line of defence? Why is this line called non-specific?
Show answerHide answer
They belong to the first line of defence. It is non-specific because it acts against any pathogen in the same way and does not need to recognise a particular antigen.
Immunisation
Immunisation stimulates the body's defence by injecting a vaccine. A vaccine contains antigens from a part of, or the whole of, a weakened or dead virus or bacterium. The antigens stimulate the immune system to make antibodies, so the body is ready if the real pathogen arrives. Babies need several vaccines on a schedule set by the Ministry of Health.
| Vaccine | Protects against |
|---|---|
| BCG | Tuberculosis |
| DTaP | Diphtheria, tetanus and pertussis |
| MMR | Measles, mumps and rubella |
| IPV | Polio |
| HPV | Human papillomavirus (girls aged 13) |
Vaccine notes: textbook Table 4.4, pp. 84–85.
Passive and active immunity
| Passive immunity | Active immunity | |
|---|---|---|
| Antibodies | Come from an external source | The body makes its own after stimulation by antigens |
| Natural | Baby receives antibodies from breast milk or across the placenta; short-lived, a few months | Person recovers from an infection; long-lasting |
| Artificial | Antiserum (a clear liquid with antibodies) is injected; fast but temporary | Vaccine with dead or weakened pathogens is injected; long-lasting |
Figures 4.5 and 4.6: textbook p. 86.
Figures 4.7 and 4.8: textbook p. 86.
Reading antibody graphs: ask where do the antibodies come from? If the line shoots up straight after an injection of antiserum and then falls, it is passive. If it rises slowly and is higher after a second dose or infection, it is active.
Worked example
Siti had chickenpox at the age of 7 and has never caught it again.Her body made its own antibodies after recovering, so it is active; it happened without an injection, so it is natural. Answer: active natural immunity.
A graph shows antibody concentration jumping to a high level straight after an injection and then falling below the immunity level within weeks. Is this active or passive immunity? Give a reason.
Show answerHide answer
Passive (artificial) immunity. The antibodies are given ready-made from outside by antiserum, so the level rises at once but is temporary because the body is not making its own.
A strong immune system
| Weakens the immune system | Strengthens the immune system |
|---|---|
| Polluted air; pesticides; stress; too much sugar; cigarette smoke | Enough sleep and rest; exercise and fresh air; not smoking and avoiding smoke; periodic health examinations |
An allergy is a response of the immune system to an allergen that is usually harmless to most people, for example mites, animal hair, dust, pollen, seafood, eggs and milk.
When people are immunised and healthy, fewer people fall ill, the cost of health care falls, work quality and quality of life improve, and diseases such as leprosy, whooping cough and tuberculosis are less likely to return. This benefits the family, society, economy and nation.
A baby is born to a mother who is immune to measles. The baby is protected for the first few months. Name the type of immunity and explain why it does not last.
Show answerHide answer
Passive natural immunity. The antibodies came from the mother (placenta, breast milk); the baby's own body did not make them, so they are used up and not replaced.