Microbes and disease. What causes what?

University Degree Medicine and Dentistry

Microbes and disease

What causes what?

These are the main agents of disease, and some of the diseases they cause:

• Viruses cause the common cold, influenza, measles, chicken pox, hepatitis B, AIDS and yellow fever.

• Bacteria cause Salmonella poisoning, impetigo, tuberculosis, diphtheria, whooping cough, typhoid, tetanus, bacterial dysentery, cholera and syphilis.

• Protoctists cause amoebic dysentery

and malaria.

• Fungi cause athlete's foot and thrush.

How do we know that microbes cause disease?

In the middle of the nineteenth century Louis Pasteur discovered that microbes in the air make food go bad. Pasteur suggested that microbes might also cause diseases.

Some years later a German doctor called Robert Koch showed that certain bacteria cause diseases such as cholera and tuberculosis. Other scientists carried out similar investigations, and by the end of the nineteenth century the particular microbes responsible for many diseases were known.

Which microbes cause diseases?

Most human diseases are caused by bacteria and viruses. However, diseases also caused by certain types of fungi, protoctists and parasitic Organisms which cause diseases are called pathogens.

When you get a disease, certain signs usually enable the doctor to td it is. The signs are called symptoms and they allow the doctor to a diagnosis.

What do germs do to us?

Germs get into the body through the nose and mouth, and through cut wounds. Once inside, they may multiply very quickly. This early stage is the incubation period, and several days or even weeks may go by before person starts feeling ill.

Germs harm us in two ways. Some of them attack and destroy our cells. Others release toxins which get into the blood. For example, the bacteria which cause cholera never leave their victim nor do they invade the cells lining it. The harm they do is caused entirely poisons which they give off.

Germs may be passed from person to person

In 1918 there was an outbreak of flu in Spain. Within a few months spread all over the world. Worldwide, over 20 million people died of it.

When a many people go down with a disease, it’s an epidemic.
If it's worldwide it’s called a pandemic

Diseases spread because the germs that cause them get passed from one person to another. A healthy individual catches the disease from someone or maybe from an animal. Such diseases are described as infectious.

Sometimes a person may have germs in his or her body without any signs of the disease. We call the person a carrier.

How are germs spread?

By droplets in the air

When you cough or sneeze, thousands of tiny drops of moisture shoot your mouth and nose. If other people breathe them in, they are likely to catch the disease. The common cold and flu spread rapidly this way, particularly crowded places.

By dust

Germs may stick to dust particles and float through the air. Eventually they settle on surfaces which may be a long way from where they came from.

Diphtheria and tuberculosis can be spread this way. People catch the disease breathing in the dust, or getting

it in their mouths from contaminated food.

By touch

Impetigo is a skin disease that breaks out in schools sometimes. You catch it by touching an infected person.

Another skin disease is athlete's foot. This can be picked up from the floor of changing rooms and showers. Diseases that are spread by touch are bribed as contagious.

By faeces

The faeces of a person with a disease may be swarming with germs. If the germs get into food or drinking water, the disease will quickly spread to other people. This is why it is important to wash your hands after going to the toilet. Epidemics of typhoid, cholera and dysentery have been caused this way.

Password

Confirm Password

I agree to the terms of service and privacy policy

Drinking water may become contaminated if sewage is not got rid of properly. This sometimes happens in poor countries.

By animals

Germs can be brought onto food by animals such as rats and mice, cockroaches and flies. Take flies, for example. These little animals are equally happy. For example, flies love to feed on dung so their proboscis may get covered in dung.

Many diseases are spread by animals which suck blood. An example is the mosquito which transmits malaria and yellow fever. Subonic plague, the Black Death of the Middle Ages, is caused by bacteria.

...

This is a preview of the whole essay

Drinking water may become contaminated if sewage is not got rid of properly. This sometimes happens in poor countries.

By animals

Many diseases are spread by animals which suck blood. An example is the mosquito which transmits malaria and yellow fever. Subonic plague, the Black Death of the Middle Ages, is caused by bacteria.

A vector is an animal that transmits a disease from one animal to another.

A dog may carry germs on it’s tounge and then lick your face. This is why it is not a good idea to let animals lick you, and wash your hands after handling then.

By blood

Diseases can be spread by blood. For example, sharing an injection needle when taking injection drugs is not a good idea because blood can be transmitted from one person to another. Two diseases which are known to be spread by blood are viral hepatitis and AIDS.

In the past, AIDS has been spread by people being given contaminated blood in a transfusion. Nowadays blood that is to be used in a transfusion is tested beforehand.

What are bacteria?

Bacteria are amongst the smallest organisms. You find them almost everywhere: in air, water, soil and inside other organisms. So they are very much part of our environment.

Many of them are useful, but some cause serious diseases.

Scientists need to grow bacteria in the laboratory. This is necessary if we are to investigate them, and find ways of fighting the diseases they cause. They can be grown on the surface of a jelly-like material called agar to which various food substances are added. The bacteria multiply in warm conditions to form colonies.

Each colony consists of thousands of bacteria. If you look at part of a colony under a good light microscope, you can

see the individual bacteria. You can see them more clearly if you look at them under the much more powerful electron microscope. Different kinds of bacteria have different shapes.

Scientists have also studied the internal structure of bacteria. They are single cells, but the cell is simpler than those of other organisms and there is no proper nucleus.

Many bacteria are good at surviving bad conditions such as drought, cold, heat, and even poisons. They can form a protective coat around themselves. They are then known as spores.

Inside the spore the bacterial cell goes to sleep. When conditions return to spore splits open and the bacterial cell comes out.

In good conditions bacteria reproduce very quickly. Their rapid reproduction makes disease bacteria difficult to overcome.

What are viruses?

Viruses are much smaller than bacteria. If you lined a type of typical virus up in a row across a page, there would be over two million of them.

Viruses can cause huge harm. All sorts of diseases in humans, animals and crop plants are caused by viruses. The virus disease which is the most talked about is AIDS, but viruses also cause less serious diseases such as flu and the common cold.

Because viruses are so small, you can only see them with an electron microscope. Different kinds of viruses have different shapes.

A virus consists of a coiled string of genes surrounded by a wall made of protein.

Viruses can only reproduce inside the cells of living organisms. If they are outside the body they usually survive for only a short time.

Inside the victim's cells, the viruses reproduce and make new viruses. The materials for making the new viruses come from the victim's cell itself. The virus kills the cell. Thousands of new viruses may be released from one cell, and they then attack more cells.

Different viruses attack different cells. For example, the common cold virus attacks cells in the nose and throat. The much more serious virus that causes AIDS attacks white blood cells.

Not all viruses reproduce as soon as they get into our cells. Some just stay there and wait, maybe for years. Then suddenly they become active and start multiplying. Until then you don't even know they are there, unless a test is carried out to show that they are.

Keeping germs out of the body

The best way of keeping germs out of the body is to get rid of them from our environment. When you rid an object of germs, it becomes sterilised. One way of sterising is by heating. However, the temperature must be high enough. Heating at 120°C for 15 minutes is enough to kill most germs. To achieve such high temperatures, a pressure cooker or autoclave must be used. Autoclaves used to sterilize pre-packed foods and hospital instruments.

Places where we particularly want to get rid of germs, such as kitchens and toilets, should be cleaned with a disinfectant. Disinfectants are chemical substances which kill germs.

One of the most germ-free places is the hospital operating theatre. A totally germ-free environment is described as aseptic.

Animals which carry germs are killed

Great efforts have been made to get rid of disease-spreading animals such rats, fleas, lice and mosquitoes. The battle against insects has been enormously by insecticides.

One of the most useful insecticides has been DDT. It was first used durring the 1939-45 war to get rid of lice. Now it has been banned in many countries because it may be dangerous in the environment. However has been extremely useful in the fight against malaria and yellow fever, both which are spread by mosquitoes.

Natural defences against disease

We are so used to the benefits of modern medicine that it is easy to forget that the human body has its own natural defences against disease. Our main defences are summarised in the picture above. Some of them prevent germs getting

into the body (e.g. skin_; others destroy germs once they have got in.

In defending the body against disease the blood plays a very important part, because it has white blood cells.

Thanks to science and medicine, our natural defenses can be helped by all sorts of artificial procedures.

Infectious individuals are isolated

Those who have a serious infectious disease, or are carriers of it, must be kept away from other people. So they are isolated until they are no longer infected. This is called being put in quarantine.

Occasionally a person entering a country is placed in quarantine because it's thought that he or she might be carrying a serious disease. This rarely happens anymore as so many infectious diseases have been brought under control. However, it is always done in Britain with animals. Cats and dogs have to be put quarantine for six months. This is to make sure that they don't bring in rabies. Rabies is a very serious disease of humans as well as animals. It is caused by a virus which attacks the brain, causing convulsions and death. Anyone who ignores the quarantine law and smuggles a pet into the country could start an epidemic.

Skin should be kept clean

The surface of the skin is very uneven, and thousands of microbes make their homes in its nooks and crannies. Some of these organisms are useful to us because they kill germs and help to protect us against disease. However, others are harmful and may cause unpleasant skin diseases. For good health it’s important to wash the skin regularly with soapy water. If you cut yourself, you open a door to germs and the cut may go septic. You put stop this happening by applying a substance such as 'surgical spirit' which kills germs. These substances are called antiseptics.

Antiseptics were discovered in the 1860s by the English surgeon Joseph Lister. In Lister's day more than half the patients in hospital who had had operations died. Many of them got a bacterial infection of their wounds called gangrene. Lister discovered that if he sprayed the patient's wound with carbolic acid during the operation, it did not go septic. So now, the number of people who died after operations was greatly reduced.

If vou cut yourself, the cut should be cleaned then dressed with sticking plaster or a bandage. The dressing stems the flow of mood, helps the blood to clot and prevents germs getting in. It also brings the cut surfaces of the skin close together, which speeds up the healing process.

Immunisation

Often you get a disease, measles say, substances called antigens on the surface of the germs cause certain white blood cells in your body to produce chemicals called antibodies. The antibodies then attack the germs, and the germs are destroyed.

Suppose some dead germs are put into your blood before you ever get a disease. Even though the germs are dead, their is still cause you to make antibodies. Antibody-producing cells then main behind and are ready and waiting attack the disease if you get it later. So receiving the dead germs has protected you against the disease. This is what doctors do when they immunise you against a disease. The first person to immunise someone against a disease was Edward Jenner. He immunised a young boy against the dreaded disease smallpox. That was in 1796. Since then immunisation has been extended to many other diseases, both viral and bacterial infections.

The dead disease, called the vaccine is put into you by a needle through your skin so the vaccine can get into your bloodstream.

Vaccines

How are vaccines made?

The aim is to produce a substance that contains the antigens of a germ. When introduced into the body, the substance must give the person immunity against the disease without causing the disease. To produce vaccines you can:

• Kill the germs by heat or chemical treatment, then use the killed germs as the vaccine, e.g. typhoid, cholera (bacterial diseases).

• Inactivate (attenuate) the germs by culturing them in special conditions so that they become harmless (non-virulent), then use them as the vaccine, e.g. tuberculosis (bacterial disease), rubella (German measles, virus disease).

• Extract the toxin from the germ and modify it chemically so that it is no longer harmful, then use that as the vaccine, e.g. diphtheria.tetanus (bacterial diseases).

• Extract antigens from the germ and use them on their own as the vaccine, e.g. influenza (virus disease).

• Use genetically engineered bacteria to mass-produce antigens for use as vaccines, e.g. hepatitis B (virus disease).

Is vaccination safe?

Think of it like this. When you are vaccinated against a disease, the doctor deliberately puts germs (or their antigens) into your body. There is no risk of you getting the disease from the vaccine.

Nevertheless, people sometimes show a slight reaction to a vaccine, and on very rare occasions the reaction may be serious. For example, it has been suggested that the measles and whooping cough vaccines may have been responsible for a few cases of brain damage - though this has never been proved.

In deciding whether or not to have a child vaccinated against a particular disease, parents have to weigh up the very slight chance of a bad reaction against the much greater chance of the child suffering from the disease if he or she is not vaccinated.

Babies are immunised against various serious diseases such as polio. Teenagers are usually immunised against tuberculosis unless a skin test shows that they are already naturally immune to it. Young girls be immunized against German measles (rubella). This is a mild disease but when a woman gets it in the early stages of pregnancy it may damage her baby.

These immunisations should protect us for the rest of our lives. However, protection against some diseases lasts for only a limited time.

In such cases you need to be given further doses of vaccine from time to keep up your protection. These are called boosters. Boosters needed for protection against diseases such as typhoid and cholera «ii people sometimes get immunised against if they are going abroad.

For immunisation to be effective, you must be given the vaccine before you get the disease. It will not cure you of the disease once you have got it.

Receiving ready-made antibodies

Tetanus is a serious bacterial disease which kills about 100 people every year Britain. The muscles, particularly those working the jaws, go into spasms - the disease is sometimes called 'lockjaw'. Tetanus germs picked up if you cut yourself with a dirty instrument such as a penknife.

Suppose you cut your finger with a dirty knife. The doctor wants to be sure you do not get tetanus. So he gives you some antibodies which have already been mack someone else, or by an animal. This is called anti-tetanus serum. The doctor injects some of this into your arm. The serum premium you getting tetanus.

Giving a person ready-made antibodies like this is useful in an err-. However, the protection does not last long. This is because the antib gradually broken down and got rid of from the body. For long-lastin;_ tion we need to make our own antibodies.

Antibiotics and drugs

Some microbes produce substances that defend them against microbes. For example, the fungus Penicillin produces a substance that acts against bacteria. The substance was discovered by Alexander Fleming in 1928, and is called penicillin.

Penicillin is just one of a very large number of drugs which today by doctors to treat people with diseases.

Pharmaceutical companies manufacture them on large scale. Some, like penicillin are obtained from microbes. They are called antibiotics, others are made by processes. They take out their effect either by killing germs or inactivating them. Over the years these substances have saved millions of lives.

Antibodies are very successful against bacteria. Unfortunately they do not work against viruses. So they will not cure people of the common cold and flu. The more serious virus diseases are treated with special anti-viral drugs.

Today, fewer and fewer people are dying from diseases.