Infectious agents and how they spread

Illness that passes from person to person

A pupil sneezes in the back row. Within a week, half the class has a cold. Nobody caught a cold from the weather. They caught it from each other.

A disease that spreads from one person to another is called an infectious disease. The living things that cause it are called infectious agents, or microbes. Most of them are far too small to see without a microscope.

In the last course we met immediate and contributory causes. For an infectious disease, the immediate cause is a microbe. Poor food, dirty water and crowding are contributory causes, because they help the microbe.

Five groups of infectious agents

All the microbes that cause disease belong to five groups.

Scientific names are written in italics. The malaria parasite is a protozoan called Plasmodium. Cholera is caused by the bacterium Vibrio cholerae, typhoid fever by Salmonella typhi, and tuberculosis by Mycobacterium tuberculosis. Anthrax is caused by Bacillus anthracis. Kala-azar comes from Leishmania, and elephantiasis from a thread-like worm called Wuchereria.

The group matters. As you will see in the next lesson, medicines that kill bacteria do nothing to viruses.

How an infection spreads

A microbe must travel from one body to another. The route it takes is called the means of spread. There are several.

Through the air. An ill person coughs or sneezes. Tiny drops carrying the microbe float in the air, and another person breathes them in. The common cold, influenza and tuberculosis spread in this way. Crowded, closed rooms make it easy.

Through water and food. Water or food may carry microbes from the waste of an ill person. A healthy person drinks or eats it and falls ill. Cholera and typhoid fever spread this way, and so does the liver disease called jaundice.

Through contact and blood. Some microbes pass in sexual contact, or in blood from an infected needle or an infected blood transfusion. The AIDS virus is the best-known example.

From mother to child. A mother can pass some microbes to her baby through the placenta before birth, or at birth. The AIDS virus can travel this way too.

By a vector. Sometimes a third animal carries the microbe from one person to the next. This carrier is called a vector.

• The **female Anopheles mosquito** carries the malaria parasite. It bites an ill person, picks up the parasite, and passes it on when it bites a healthy person.
• The **Aedes mosquito** spreads the dengue virus.
• The housefly lands on waste, then on food. Its feet carry the germs of cholera, typhoid fever and diarrhoea.

A mosquito that bites a healthy person has not caused the illness by the bite. It only delivered a microbe that came from someone else.

Not every touch spreads every disease

Each disease spreads in its own ways, and in no others. The AIDS virus does not spread by a handshake, a hug, shared food or a mosquito bite. A common cold does not spread by blood. Knowing the exact route tells us how to stop it. A person who believes the wrong route will take the wrong care.

A cold in a class of 12

Here is a model of a cold in a class of 12 pupils. Some pupils start ill. Each healthy pupil has a chance of catching the cold from each ill pupil. Move the sliders and watch the red part of the bar grow.

Set the chance to $0$ % and nobody new catches it. Set it to $100$ % with two ill pupils, and the whole class is ill. More ill pupils at the start make it spread faster, because each healthy pupil meets more of them.

Where in the body does it show up?

Once inside, many microbes settle in a particular organ or tissue. The signs of the illness depend on where they settle. A microbe that does not damage an organ does not cause signs in it.

• Lungs. Tuberculosis bacteria and many viruses settle in the lungs. The signs are coughing and difficulty in breathing, and sometimes blood in the cough.
• Liver. The virus that causes jaundice settles in the liver. The liver stops working well, and the skin and eyes turn yellow.
• Brain. The virus that causes Japanese encephalitis, spread by mosquitoes, reaches the brain. The signs are headache, high fever, vomiting and, if it is serious, loss of consciousness.
• Blood. The malaria parasite goes first into the liver. It then enters the red blood cells and breaks them open, which causes fever with shivering.
• The immune system. The AIDS virus settles in the white blood cells that protect us. We return to this in the third lesson.

Some signs appear whatever the microbe is. The body fights back with inflammation, the redness, pain and swelling around a damaged tissue. It also raises its temperature, and this fever helps to fight microbes. A fever tells us that the body is fighting something. It does not tell us which microbe it is.

Try it in Python

This program follows a cold in the same class of 12 pupils. The line random.seed(5) makes the random numbers repeat, so the answer is the same every time.

The numbers grow slowly at first and then quickly. Each new ill pupil passes the cold on to others, so the spread speeds up. With a different seed the exact numbers change, but the pattern stays the same.

Practise with new numbers

Common mistakes

• Thinking all microbes are bacteria. Viruses, bacteria, fungi, protozoa and worms all cause disease. They are different groups.
• Blaming the mosquito's bite alone. The mosquito is only the carrier. The microbe came from an ill person.
• Believing that AIDS spreads by touch. It does not spread by handshakes, hugs, shared food or mosquito bites.
• Thinking a fever shows which microbe is present. Many diseases cause fever. The doctor needs more clues.
• Forgetting where the microbe settles. The organ it damages explains the signs. Tuberculosis shows in the lungs, jaundice in the liver.

Remember these

• Infectious agents: viruses, bacteria, fungi, protozoa and worms.
• Spread: air, water and food, contact and blood, mother to child, and vectors.
• A vector is a carrier. The female Anopheles mosquito carries malaria. The housefly carries germs from waste to food.
• The organ the microbe settles in decides the signs: lungs in tuberculosis, liver in jaundice, brain in encephalitis.
• Inflammation and fever are signs that the body is fighting the infection.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the five groups of infectious agents. Give one disease for each.

2. How does cholera spread, and how does malaria spread?

3. A person with tuberculosis keeps coughing. In which organ have the bacteria settled?

4. Why is the housefly called a vector of diarrhoea?

5. One ill pupil meets $50$ healthy pupils, and each has a $10$ % chance of catching the illness. How many pupils are expected to catch it?