S4.4.2 Chapter 2

How diseases spread

How does a germ get from one person to another when nobody even touches?

Your guess

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

Your guess

A Germs pass only when two people touch.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Germs travel only through the air.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Germs can travel in the air, in water, in food, on hands, in blood and on insects.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how diseases spread by air, water, food, touch, blood and insects
Explain how mosquito nets, clean water and toilets cut disease

In the last chapter you met pathogens. A pathogen causes disease only when it gets into a body and multiplies there. This is an infection. For a communicable disease to spread, a pathogen must leave one body and get into another. The way it travels is its route.

Draw it first

A pathogen on the move

1 A pathogen leaves an infected person, for example in a cough or in waste from the body. It then travels by one of the routes.
2 It gets into another person, who is now infected.
3 It multiplies in that person, who may now pass it on. Cutting any step stops the spread.

There are six main routes. The picture shows all six. A rod stands for a pathogen. Rods are not to scale: real pathogens are far smaller.

The six routes. A rod is a pathogen, drawn much too large to see.

The air route. A person with influenza, measles or tuberculosis breathes out tiny droplets of liquid when they cough, sneeze or talk. The droplets carry pathogens. A person nearby breathes them in. Covering the mouth when coughing, and fresh air in a room, cut this route.

The water route. Some pathogens leave the body in faeces. If faeces get into a river, a pool or a well, the water is contaminated, which means it carries pathogens. Cholera spreads this way. Anyone who drinks the water can be infected.

The food route. Pathogens can get onto food from dirty hands, from dirty water or from flies. Bacteria then multiply on the food, fastest when it is warm. Cooking food fully, keeping it covered and keeping it cool cut this route.

The touch route. Some pathogens pass when skin touches skin, or when a hand touches a surface and then your face. Athlete's foot and ringworm spread this way, and so do many colds. Washing hands cuts this route.

The blood route. A few pathogens, such as HIV, are in the blood of an infected person. They pass when infected blood gets into another person's body, for example through a shared needle. HIV also passes in some other body fluids, as chapter 6 explains. These pathogens do not pass by a touch, a hug or a shared cup.

The insect route. An animal that carries a pathogen from one host to another is a vector. The mosquito is the vector of malaria. It bites an infected person and takes up pathogens. Later it bites another person and passes them on.

Each route can be cut. Good hygiene, which means habits that keep you and your surroundings clean, blocks many routes at once.

See it three ways

As a picture

In words

Each control cuts one route. A toilet keeps faeces away from water, food and hands. Clean water has no pathogens in it. Soap and water lift pathogens off the hands. A net keeps mosquitoes off a sleeping person.

Clean water is treated or protected so that faeces cannot get into it. Water that may be contaminated can be made safer by boiling it. Toilets and drains take faeces away from people, water and food. Together these cut the water route, which is why cholera is now rare where they are in place.

Hand washing needs soap and water. Soap lifts dirt and pathogens off the skin, and the water rinses them away. Water alone removes less. The best times to wash are after using the toilet and before cooking or eating.

In places with malaria, a bed net hangs over the bed. Many of the mosquitoes that carry malaria bite at night. The net is a barrier between the mosquito and the sleeper. Removing standing water also helps, because mosquitoes lay their eggs in it.

The net blocks the mosquito, as long as it has no holes and is tucked in all round.

The same bed with no net. Nothing stands between the mosquito and the sleeper.

We can measure how well a control works. The bar chart shows cases of cholera in a village each year, before and after a clean well was built. The numbers are an example, made up to show the method.

Cases of cholera in one village in the year before and the year after a clean well. Example numbers.

Worked example

Use the bar chart. By what percentage did the cases of cholera fall after the well was built?

Read the bars. Before: 40 cases. After: 10 cases.
The fall is 40 − 10 = 30 cases.
Percentage fall = fall ÷ before × 100 = 30 ÷ 40 × 100 = 75.

Answer: 75%

Worked example

The village has 800 people. How many cases of cholera were there for every 1 000 people in the year before the well?

There were 40 cases among 800 people.
Divide the cases by the people: 40 ÷ 800 = 0.05.
Multiply by 1 000: 0.05 × 1 000 = 50.

Answer: 50 cases

Try it

Watch a touch spread

You need: a little flour on a plate, a cup, a tray or a big sheet of paper, a bowl of water and soap, a clean cloth.

Be safe: Keep flour away from your eyes, nose and mouth, and do not taste it. Do not do this if flour makes you ill. Clean up with a damp cloth, not a dry brush that lifts dust. An adult should be near.

Work over the tray. Rub a pinch of flour on both hands. The flour stands for pathogens.
Pick up the cup, then touch a door handle or a table. Look at what you leave behind.
Dip your hands in water only. Rub them for a few seconds and look at them.
Now wash with soap and water. Rub the palms, backs and between the fingers for about 20 seconds, then rinse. Look again.

What you will see: The flour spreads to everything you touch. Water alone leaves some flour on your hands. Soap and water removes almost all of it. Flour is only a model. Real pathogens are too small to see, so wash even when your hands look clean.

If you cannot try it, this is what happens.

A wrong idea many people have

Malaria is caught from bad air near a swamp, or from a sick person who breathes on you.

Why it feels right: People get malaria most near still water. A person with malaria has a fever, which looks like a disease that spreads through the air.

The test that breaks it: Put a net over the bed in the same place and fewer people get malaria, even though the air is the same. A sick person cannot give you malaria by breathing on you or touching you. Nearly every case needs the mosquito.

What is true: Malaria is spread by the bite of an infected mosquito, which breeds in still water. It does not travel through the air or pass by touch.

Key words

infection: a pathogen that has got into the body and is multiplying.
route: the way a pathogen travels from one person to another.
droplets: tiny drops of liquid breathed out in a cough or sneeze. They can carry pathogens.
contaminated: carrying pathogens or other harmful things. Water can be contaminated.
vector: an animal that carries a pathogen from one host to another.
hygiene: habits that keep you and your surroundings clean and so cut the spread of disease.

Back to your guess

The answer is C: Germs can travel in the air, in water, in food, on hands, in blood and on insects.

A pathogen must leave one body and get into another. There are six main routes: air, water, food, touch, blood and insects. Each route can be cut, and that is how clean habits and nets protect people.

Keep it

Copy this into your exercise book.

Pathogens spread by air, water, food, touch, blood and insects. Each route can be cut: covered coughs, clean water, toilets, cooked food, soap and water, and bed nets.

Draw: Six small boxes, one for each route, each with the control that cuts it written under it.