Chapter 1 of 8

All 8 chapters
  1. 1Kinds of disease, readunderstood
  2. 2How diseases spread, readunderstood
  3. 3The body's defences, readunderstood
  4. 4Vaccination, readunderstood
  5. 5Antibiotics and medicines, readunderstood
  6. 6Malaria, HIV and tuberculosis, readunderstood
  7. 7Lifestyle and disease, readunderstood
  8. 8Sexual health, readunderstood

S4.4.1 Chapter 1

Kinds of disease

Why can I catch a cold from a friend, but nobody can catch heart disease from me?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Sort diseases into communicable and non-communicable
  • Name the four kinds of pathogen with one disease each

A disease is a condition that stops part of the body from working as it should. Some diseases pass from one person to another. Others never do. This chapter sorts diseases into those two groups. Then it meets the tiny things that cause the first group.

A pathogen is a germ that causes disease. A single pathogen is too small to see without a microscope. When enough of them get into a body, they can make a person ill.

Draw it first

How a pathogen makes you ill

  1. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: A pathogen gets into the body; It multiplies; It damages cells or makes poisons; You feel ill.A pathogen gets into the bodyIt multiplies

    1 A pathogen gets into the body, for example in air that is breathed in. It multiplies, so one pathogen becomes many.

  2. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: A pathogen gets into the body; It multiplies; It damages cells or makes poisons; You feel ill.A pathogen gets into the bodyIt multipliesIt damages cells or makespoisons

    2 It damages cells or makes poisons. A poison made by a pathogen is called a toxin.

  3. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: A pathogen gets into the body; It multiplies; It damages cells or makes poisons; You feel ill.A pathogen gets into the bodyIt multipliesIt damages cells or makespoisonsYou feel ill

    3 Then you feel ill, for example with a fever or a rash.

A communicable disease is caused by a pathogen and can pass from one person, or animal, to another. It is also called an infectious disease. Measles, influenza, cholera and malaria are communicable diseases.

A non-communicable disease cannot be passed from one person to another. Its causes include the genes you inherit, too little of a vitamin in your food, and tobacco smoke. Heart disease, sickle cell disease and scurvy are examples. Scurvy comes from too little vitamin C. An inherited disease passes from parents to a child in their genes, not by infection, so it is still non-communicable.

A table with the columns Disease, Passes on?. measles, yes. cholera, yes. heart disease, no. scurvy, no.DiseasePasses on?measlesyescholerayesheart diseasenoscurvyno
Yes means the disease can pass from one person to another, so it is communicable. No means it is non-communicable.

There are four kinds of pathogen. They are very different from each other. We meet them in order, starting with the one that most people have heard of.

Bacteria are single cells with no nucleus. Each has a cell wall and a loop of DNA. Most bacteria are harmless or helpful. A few cause diseases such as tuberculosis and cholera.

A virus is not a cell. It is genetic material inside a coat of protein. A virus can multiply only inside the living cells of another organism. Measles, influenza and HIV are caused by viruses.

Fungi include moulds and yeast. Some fungi live on the skin and cause athlete's foot and ringworm. Ringworm is not a worm. It is caused by a fungus.

A protist is a single celled living thing with a nucleus. Most protists are harmless. One kind causes malaria, a disease that a mosquito carries from person to person.

Four panels, stacked, one for each kind of pathogen, with a note under them. The panels are named bacterium, virus, fungus and protist. The bacterium has the labels cell wall and loop of DNA. The virus has the labels protein coat and genetic material. The fungus has the label hyphae. The protist has the labels nucleus and cell membrane. The note says: Not drawn to the same scale. Usually, from the smallest: virus, bacterium, then fungus or protist cell.bacteriumcell wallloop of DNAvirusprotein coatgenetic materialfungushyphaeprotistnucleuscell membraneNot drawn to the same scale.Usually, from the smallest:virus, bacterium, thenfungus or protist cell.
The four kinds of pathogen. Sizes vary a lot inside each kind. Hyphae are the thin threads that make up a fungus.

Most viruses are between about 20 nm and 300 nm across. Most bacteria are about 1 µm to 5 µm long. One micrometre is 1 000 nm. So a virus is usually many times smaller than a bacterium.

See it three ways

As a picture

A table with the columns Pathogen, One disease. bacterium, tuberculosis. virus, measles. fungus, athlete's foot. protist, malaria.PathogenOne diseasebacteriumtuberculosisvirusmeaslesfungusathlete's footprotistmalaria

In words

Each kind of pathogen causes its own diseases. Learn one disease for each kind and you can sort many others.

Bacteria multiply by dividing in two. In warm, moist food, some kinds divide about every 20 minutes. The graph shows one bacterium that does this. The number doubles with each division, so it grows faster and faster.

A line graph. The x axis is minutes, from 0 to 120 in steps of 20. The y axis is bacteria, from 0 to 64 in steps of 16. The points are at 0 minutes and 1 bacteria, 20 minutes and 2 bacteria, 40 minutes and 4 bacteria, 60 minutes and 8 bacteria, 80 minutes and 16 bacteria, 100 minutes and 32 bacteria, 120 minutes and 64 bacteria. The line bends upward, faster and faster.020406080100120016324864minutesbacteria
One bacterium that divides every 20 minutes, with none dying. The line bends upward because each division doubles the number.

Worked example

A bacterium divides in two every 20 minutes. Use the graph to find how many bacteria there are after 100 minutes.

  1. Find 100 minutes on the x axis, and go up to the line.
  2. Read across to the y axis. The line is at 32.
  3. Check by doubling. 100 ÷ 20 = 5 divisions, so 1 × 2 × 2 × 2 × 2 × 2 = 32.

Answer: 32 bacteria

Worked example

Mei puts 5 bacteria on some warm food. Each one divides every 20 minutes. How many bacteria are there after 3 hours, if none die and the food does not run out?

  1. 3 hours = 3 × 60 = 180 minutes.
  2. Divisions: 180 ÷ 20 = 9.
  3. Each division doubles the number. After 9 divisions it is multiplied by 512.
  4. Number of bacteria: 5 × 512 = 2 560.

Answer: 2 560 bacteria

This cannot go on for ever. Real food runs out, and waste builds up. The sum shows why a few bacteria can make a person ill in a day, and why the body needs defences. The next chapters take up both.

Try it

Sort the diseases

You need: a sheet of paper, a pencil.

  1. Tear the paper into eight slips.
  2. Write one disease on each slip: measles, cholera, heart disease, malaria, scurvy, influenza, athlete's foot, sickle cell disease.
  3. Make two piles. Put a disease in the first pile if a pathogen causes it and it can pass between people. Put the rest in the second pile.
  4. For each slip in the first pile, write the kind of pathogen: bacterium, virus, fungus or protist.

What you will see: Five slips go in the first pile and three in the second. In the first pile, a bacterium causes cholera. Viruses cause measles and influenza. A fungus causes athlete's foot, and a protist causes malaria.

If you cannot try it, this is what happens.
A table with the columns Slip, Pile. measles, first. cholera, first. malaria, first. influenza, first. athlete's foot, first. heart disease, second. scurvy, second. sickle cell disease, second.SlipPilemeaslesfirstcholerafirstmalariafirstinfluenzafirstathlete's footfirstheart diseasesecondscurvysecondsickle cell diseasesecond

A wrong idea many people have

All germs make us ill.

Why it feels right: We mostly hear about germs when someone is ill, so they seem to be always harmful.

The test that breaks it: Yoghurt is made with the help of bacteria. Bread dough rises because yeast, a fungus, grows in it. Bacteria live in your gut all your life and cause no harm.

What is true: Only a small share of bacteria, fungi and other microbes are pathogens. Most are harmless, and many are useful.

Key words

disease
a condition that stops part of the body working as it should.
pathogen
a germ that causes disease.
communicable disease
a disease caused by a pathogen that can pass from one person to another.
non-communicable disease
a disease that no pathogen causes, so nobody catches it from another person.
bacteria
single cells with no nucleus. A few cause disease.
virus
genetic material in a protein coat. It multiplies only inside living cells.
fungi
moulds and yeast. Some cause skin diseases.
protist
a single celled living thing with a nucleus. One kind causes malaria.

Back to your guess

The answer is B: Some diseases are caused by germs and pass between people. Others have different causes and cannot be caught.

Only a disease caused by a pathogen can pass from person to person. Heart disease has other causes, so nobody can catch it. And a pathogen does not need touch: some travel in air, in water or in food.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which of these is a communicable disease?

    Tap your answer.

    1. Not this one. Heart disease has causes such as diet and smoking. Nobody can catch it from another person. Try another.

    2. Not this one. It is inherited through genes. It is not caused by a pathogen, so it cannot be caught. Try another.

    3. Yes, that is right. Measles is caused by a virus, and it passes from one person to another. The other three have causes that cannot be caught.

    4. Not this one. Scurvy comes from too little vitamin C in the food. It is not caused by a pathogen. Go to S3.3.3. Try another.

    See the answer

    Measles

    Measles is caused by a virus, and it passes from one person to another. The other three have causes that cannot be caught.

    • If you chose A: Heart disease has causes such as diet and smoking. Nobody can catch it from another person.
    • If you chose B: It is inherited through genes. It is not caused by a pathogen, so it cannot be caught.
    • If you chose D: Scurvy comes from too little vitamin C in the food. It is not caused by a pathogen. Go to S3.3.3.

  2. What does non-communicable mean?

    Tap your answer.

    1. Yes, that is right. Non-communicable describes a disease that is not passed on. Its causes are things such as genes, diet and tobacco smoke.

    2. Not this one. Many non-communicable diseases can be treated. The word is about whether the disease passes between people. Try another.

    3. Not this one. A disease caused by a virus is communicable. Non-communicable diseases are not passed on. Try another.

    See the answer

    It cannot be passed from one person to another.

    Non-communicable describes a disease that is not passed on. Its causes are things such as genes, diet and tobacco smoke.

    • If you chose B: Many non-communicable diseases can be treated. The word is about whether the disease passes between people.
    • If you chose C: A disease caused by a virus is communicable. Non-communicable diseases are not passed on.

  3. Look at the picture of two pathogens. Which one is not a cell?

    Two panels, stacked, with a note under them. The panels are named bacterium and virus. The bacterium has the labels cell wall and loop of DNA. The virus has the labels protein coat and genetic material. The note says: Not drawn to the same scale. Usually, from the smallest: virus, then bacterium.bacteriumcell wallloop of DNAvirusprotein coatgenetic materialNot drawn to the same scale.Usually, from the smallest:virus, then bacterium.
    Tap your answer.

    1. Not this one. A bacterium is a single cell. It has a cell wall and a loop of DNA. Try another.

    2. Yes, that is right. A virus is genetic material inside a protein coat, and it has no cell parts. It can multiply only inside the living cells of another organism.

    3. Not this one. Only the bacterium is a cell. A virus is genetic material in a protein coat. Try another.

    See the answer

    The virus

    A virus is genetic material inside a protein coat, and it has no cell parts. It can multiply only inside the living cells of another organism.

    • If you chose A: A bacterium is a single cell. It has a cell wall and a loop of DNA.
    • If you chose C: Only the bacterium is a cell. A virus is genetic material in a protein coat.

  4. Which kind of pathogen causes malaria?

    Tap your answer.

    1. Not this one. Bacteria cause diseases such as tuberculosis and cholera. Malaria has a different cause. Go to S3.1.8. Try another.

    2. Not this one. Measles, influenza and HIV are caused by viruses. Malaria is not. Try another.

    3. Not this one. Fungi cause skin diseases such as athlete's foot. Malaria is not a fungus disease. Try another.

    4. Yes, that is right. Malaria is caused by a single celled living thing with a nucleus, which is a protist. A mosquito carries it from person to person.

    See the answer

    A protist

    Malaria is caused by a single celled living thing with a nucleus, which is a protist. A mosquito carries it from person to person.

    • If you chose A: Bacteria cause diseases such as tuberculosis and cholera. Malaria has a different cause. Go to S3.1.8.
    • If you chose B: Measles, influenza and HIV are caused by viruses. Malaria is not.
    • If you chose C: Fungi cause skin diseases such as athlete's foot. Malaria is not a fungus disease.

  5. Which pair of disease and pathogen is right?

    Tap your answer.

    1. Not this one. Measles is caused by a virus. Try another.

    2. Not this one. Cholera is caused by a bacterium. Try another.

    3. Yes, that is right. Tuberculosis is caused by a bacterium. Measles is caused by a virus, cholera by a bacterium and athlete's foot by a fungus.

    4. Not this one. Athlete's foot is caused by a fungus that lives on the skin. Try another.

    See the answer

    Tuberculosis: a bacterium

    Tuberculosis is caused by a bacterium. Measles is caused by a virus, cholera by a bacterium and athlete's foot by a fungus.

    • If you chose A: Measles is caused by a virus.
    • If you chose B: Cholera is caused by a bacterium.
    • If you chose D: Athlete's foot is caused by a fungus that lives on the skin.

  6. Which statement about bacteria is true?

    Tap your answer.

    1. Yes, that is right. Only a small share of bacteria are pathogens. Many live in the gut or in soil and do no harm.

    2. Not this one. Most bacteria do no harm, and some are useful. Yoghurt is made with their help. Go to S3.1.8. Try another.

    3. Not this one. Viruses are usually much smaller than bacteria. Try another.

    See the answer

    Most are harmless or helpful, and a few cause disease.

    Only a small share of bacteria are pathogens. Many live in the gut or in soil and do no harm.

    • If you chose B: Most bacteria do no harm, and some are useful. Yoghurt is made with their help. Go to S3.1.8.
    • If you chose C: Viruses are usually much smaller than bacteria.

  7. How can a virus multiply?

    Tap your answer.

    1. Not this one. A bacterium is a cell and divides in two. A virus is not a cell, so it cannot divide. Try another.

    2. Yes, that is right. A virus has no cell parts of its own. It uses the living cells of another organism to make more viruses.

    3. Not this one. Bacteria and fungi can grow in food. A virus needs a living cell to multiply. Try another.

    4. Not this one. A virus cannot multiply in water, clean or not. It needs a living cell. Try another.

    See the answer

    Only inside the living cells of another organism.

    A virus has no cell parts of its own. It uses the living cells of another organism to make more viruses.

    • If you chose A: A bacterium is a cell and divides in two. A virus is not a cell, so it cannot divide.
    • If you chose C: Bacteria and fungi can grow in food. A virus needs a living cell to multiply.
    • If you chose D: A virus cannot multiply in water, clean or not. It needs a living cell.

  8. A bacterium divides in two every 20 minutes. How many bacteria come from one bacterium after 2 hours, if none die?

    See the answer

    64 bacteria

    2 hours = 120 minutes. 120 ÷ 20 = 6 divisions. The number doubles 6 times: 2 × 2 × 2 × 2 × 2 × 2 = 64.

  9. 50 people ate the same meal. 20 of them became ill with an infection caused by bacteria. What percentage of the people became ill?

    See the answer

    40%

    20 ÷ 50 × 100 = 40.

Keep it

Copy this into your exercise book.

A disease that a pathogen causes and that passes between people is communicable. Other diseases are non-communicable. The pathogens are bacteria, viruses, fungi and protists. Tuberculosis, measles, athlete's foot and malaria are one example of each.

Draw: Four boxes named bacteria, virus, fungus and protist, each with one disease written beside it.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.