Microbes and disease
It is unusual for a scientific discovery to come from nowhere. Usually it is made after the scientist has noted many observations and gathered together many facts. We can look at how observation, fact gathering, reasoning and experimentation led to a major discovery which has affected the lives of everyone on the planet.

There is a disease called smallpox, which was once widespread throughout the world. It is caused by a virus. In a smallpox infection the breathing system is first attacked, then the virus moves into the skin and causes scabs to develop. In the scabs a creamy liquid called pus is produced, which teems with viruses. The body reacts to the invasion by producing a fever. For every hundred people who caught a strong form of smallpox thirty died. Those people who recovered were often terribly scarred by the scabs. Once an outbreak of smallpox developed in a village or town it could spread quickly as the viruses escaped into the air from the scabs and were breathed in by other people. However, some forms of the disease were milder and did less damage and people who caught smallpox and survived never caught it again.

Over two thousand years ago in China, people had noticed that there was a chance to survive smallpox and there was also a chance that you might not suffer too dangerous an attack. With this in mind they took pus from infected people and rubbed it into the skin of healthy young people who had not had the disease. They reasoned that if the person survived the attack they had been given, they need never fear smallpox again. While some people only developed mild forms and became immune to smallpox, others received a strong form, which killed them. As there was no better way of making a person immune, the practice which was called variolation, was continued and by the eighteenth century had spread to Europe.

In the late eighteenth century there lived a doctor called Edward Jenner. He had his practice in the English countryside. He knew about variolation but he also picked up a useful fact while treating one of his patients. She was a milkmaid and had had a disease called cowpox. This disease was found in cows and was passed onto milkmaids as they collected milk from the cow's udder. Cowpox was a much milder disease than smallpox. Usually all that developed in an attack of cowpox were sores like warts on the hands and forearms. Jenner discovered through his discussion with the milkmaid that a person who had had cowpox never caught smallpox. When he asked other local people, he found they also believed the milkmaid's statement to be true.

Jenner decided to test the idea using a modified process of variolation. He chose a boy of eight called James Phipps to help him. Jenner's plan was to give James the cowpox disease and when he recovered show that James was immune to smallpox. Jenner scratched James's arm then collected some pus from a cowpox scab. He rubbed the pus into the scratches and kept a careful watch on James over the following weeks. James was attacked by the cowpox disease but he quickly recovered. Jenner then took some pus from a smallpox victim and put it into the scratches. He kept a careful watch on James's health and was relieved to find that James did not become ill. The attack of cowpox had made him immune to smallpox just as the milkmaid believed.

At the time it was usual for doctors to be familiar with Latin and Jenner discovered that the Latin word for cowpox was vaccinia. He used this word to help him describe his new process of fighting smallpox. He called his process vaccination. When Jenner told other doctors about his discovery they began to vaccinate people against smallpox. Today vaccination has wiped out the disease due to Jenner's careful observation, fact gathering and experimentation. If you are keen on science for a career you will need to develop similar skills to Jenner. Then you might make a discovery that will change the world for the better.

Is the word 'vaccination' just used in connection with smallpox?
It was originally. Today it is used to describe the process in which people are made immune to any disease.

What other diseases are people vaccinated against?
In the United Kingdom there is a programme to make all children immune to diphtheria, measles, mumps, polio, rubella, tetanus, tuberculosis and whooping cough.

Are people vaccinated against them all at once?
No. They are vaccinated seven times during the first fifteen years of life. At most vaccinations they are vaccinated against several diseases.

What is in a vaccine?
There are three kinds of vaccine. The first kind of vaccine, such as the vaccines for polio and tuberculosis, are made from microbes, which have been weakened. The second kind of vaccine, such as the vaccine for whooping cough, is made by dead microbes and the third kind of vaccine, such as the vaccine for tetanus, is made from poisons created by the microbe but the poison has been rendered harmless.

How does a vaccine work?
There are features on the body of a harmful microbe called antigens. When the harmful microbe enters the body, the antigens are detected and the body makes antibodies to attack them. When a microbe is attacked by antibodies, it can no longer cause disease. The substance in the vaccine has the antigen to make the body produce antibodies but the substance does not cause a disease. This means that the body gets to build up its defence against the disease without actually getting the disease. In later life, if a vaccinated person is invaded by a disease-causing microbe, the body is ready for it and produces antibodies quickly so the disease does not develop.

Won't one vaccine stop all diseases?
No. Each disease-causing microbe has a special kind of antigen and the body must make a special kind of antibody to match it. This means that you must be vaccinated with different microbes so that you can make different antibodies to protect you from different diseases.

What is a booster?
Some vaccinations can make your body immune to a disease for your lifetime but others only give protection for a number of years. A booster is an extra vaccination that is given to keep the body protected from certain diseases such as tetanus.

If a person has a disease what can be done to help them?
If the disease is caused by a bacterium an antibiotic can help. This will destroy the harmful bacteria in the body and as the number of harmful bacteria falls the person will start to recover. Antibiotics do not destroy viruses. There are some antiviral drugs which stop a viral infection. The antiviral drug prevents the virus reaching the chemicals it needs that are inside the body cells.

Are all diseases caused by microbes?
No. Some diseases such as heart disease are caused by lifestyle. For example a person who does not take exercise, has a large amount of fat in the diet and is overweight may develop heart disease. Smoking cigarettes increases the chances of developing lung cancer.

Can some diseases be carried by animals?
Yes, they can. The bubonic plague is carried by two animals - the rat and the rat flea. A rat which is infected with the bacterium that causes the plague, carries the disease from place to place. The flea on the rat carries the disease from the rat to people.

How does the rat flea transmit the disease?
A flea feeds on blood. It has a special mouth, which acts like a needle and pierces the skin to reach the blood. When a rat flea feeds on a diseased rat it takes in the disease-causing microbe. If it hops from a rat to a person and feeds on the person it passes on the disease-causing microbe to the person.

Are fleas the only insects which transmit disease?
No. Malaria is spread by a certain kind of mosquito. The microbe causing malaria is a protozoan. It spends part of its life cycle in the body of the mosquito and part in the body of a person. Sleeping sickness is spread by the tsetse fly. This disease is also caused by a protozoan.