Pathogens are microorganisms that cause disease. The body has several defence mechanisms to prevent pathogens from entering the body and reproducing there. The immune system can destroy pathogens that manage to enter the body. New medical treatments and drugs must be tested before their use.
Pathogens are organisms that cause disease. They include microorganisms such as bacteria, viruses, fungi and protozoa.
- Bacteria - Clolera
- Viruses - Infuenza (flu)
- Fungi - Athlete's Foot
- Protozoa - Malaria
Parasites are organisms that live on or in a host organism. The parasite benefits from this arrangement, but the host suffers as a result.
For example, tapeworms are parasites. They live inside another animal, attaching themselves to the host’s gut and absorbing its food. The host loses nutrition, and may develop weight loss, diarrhoea and vomiting. Parasites do not usually kill the host, as this would cut off their food supply.
Malaria is a disease caused by a protozoan, a type of single-celled organism. The malaria parasite is spread from person to person by mosquitoes. These insects feed on blood and the malaria parasite is passed on when the mosquito takes a meal. Organisms that spread disease, rather than causing it themselves, are called vectors. The mosquito is the vector for malaria.
The life cycle of a pathogen
The way in which vectors spread the pathogen. This knowledge gives scientists ideas about how the spread of a disease might be controlled. If the life cycle of a pathogen can be broken, eventually all the individuals of that pathogen will die out, leaving a disease-free population.
The spread of malaria can be controlled by avoiding contact with the vector. One way to do this is to avoid being bitten by mosquitoes by:
- using mosquito netting at windows, doors and around beds.
- The mosquitoes may be killed using insecticides.
- The parasite itself can be killed by giving infected people drugs such as Lariam.
Infectious and non-infectious diseases
Some diseases are not caused by pathogens and so are not infectious.
- scurvy is caused by vitamin C deficiency
- anaemia is caused by iron deficiency
- diabetes and cancer are disorders of the body.
- Some disorders are inherited, such as red-green colour vision deficiency.
A cancer happens when cells begin to divide out of control. They form tumours that can sometimes be felt as an unusual lump in the body. Diet and lifestyle can increase the risk of developing certain cancers.
- smoking increases the risk of lung cancer
- using sunscreen reduces the risk of skin cancer
- eating more fruit and vegetables reduces the risk of bowel cancer.
Tumours can be benign or malignant, benign tumours grow slowly and are usually harmless – warts are benign tumours malignant tumours often grow more quickly and may spread throughout the body.
The body has several defences against pathogens so that we do not fall ill with the diseases they cause.
The skin covers the whole body. It protects the body from physical damage, microbe infection and dehydration. Its dry, dead outer cells are difficult for microbes to penetrate, and the sebaceous glands produce oils that help kill microbes.
If microorganisms get into the body through a cut in the skin, the most important thing to do is close the wound quickly so that no more microorganisms can enter. A scab does just that. The blood contains tiny structures called platelets, and a protein called fibrin. A scab is basically platelets stuck in a fibrin mesh.
The respiratory system is protected in several ways. Nasal hairs keep out dust and larger microorganisms. Sticky mucus traps dust and microbes, which are then carried away by cilia. These are tiny hairs on the cells that line the respiratory system.
Hydrochloric acid in the stomach kills harmful microorganisms that might be in the food or drink that we swallow.
Once inside the body, pathogens reproduce. Viruses reproduce inside cells and damage them, while escaping to infect more cells. Bacteria produce toxins - poisons. Cell damage and toxins cause the symptoms of infectious diseases. Once pathogens enter the body, the immune system destroys them. White blood cells are important components of the immune system.
White blood cells
White blood cells can:
- engulf pathogens and destroy them
- produce antibodies to destroy pathogens
- produce antitoxins that neutralise the toxins released by pathogens.
Pathogens contain certain chemicals that are foreign to the body, called antigens. Some white blood cells can make antibodies. These are proteins that have a chemical 'fit' to a certain antigen. When a white blood cell with the appropriate antibody meets the antigen, it reproduces quickly and makes many copies of the antibody to neutralise the pathogen.
Once you have been infected with a particular pathogen and produced antibodies against it, some of the white blood cells remain. If you become infected again with the same pathogen, these white blood cells reproduce very rapidly and the pathogen is destroyed. This is active immunity because you make your own antibodies.
Sometimes you may be treated for infection by an injection of certain antibodies from someone else. This is passive immunity because you receive antibodies, rather than make them yourself
Drugs are substances that cause changes to the body. Antibiotics are drugs that kill bacteria, but not viruses. Antivirals are drugs that prevent viruses reproducing.
New medical drugs have to be tested to ensure that they work, and are safe, before they can be prescribed. There are three main stages of testing.
- The drugs are tested using computer models and human cells grown in a laboratory. Many substances fail this test because they damage cells or do not seem to work.
- Drugs that pass the first stage are tested on animals. In the UK, new medicines have to undergo these tests. But it is illegal to test cosmetics and tobacco products on animals. A typical test involves giving a known amount of a substance to the animals, then monitoring them carefully for any side-effects.
- Drugs that have passed animal tests are used in clinical trials. They are tested on healthy volunteers to check that they are safe. Very low doses of the drug are given to begin with. If there are no problems, further clinical trials are done to find the optimum dose for the drug.
Double blind trials
It is important to be certain that a drug really does have positive effects, rather than people feeling better simply because they expect to feel better if they take a medicine. This is called the ‘placebo effect’. Double blind trials aim to minimise the placebo effect. Some patients are given the drug while others are given a placebo. A placebo is designed to appear exactly the same as the drug itself, but it does not actually contain any of the drug. The doctors and patients are not told who have received the drug and who have received the placebo until the trial is over.
People can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens.
Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body. Vaccines can contain:
- live pathogens treated to make them harmless
- harmless fragments of the pathogen
- toxins produced by pathogens
- dead pathogens.
These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies against the pathogen.
Because the vaccine contains only a weakened or harmless version of a pathogen, the vaccinated person is not in danger of developing disease - although some people may suffer a mild reaction. If the person does get infected by the pathogen later, the required white blood cells are able to reproduce rapidly and destroy it.
Over time, bacteria can become resistant to certain antibiotics. MRSA is methicillin-resistant Staphylococcus aureus. It is very dangerous because it is resistant to most antibiotics. To slow down or stop the development of other strains of resistant bacteria, we should:
- avoid the unnecessary use of antibiotics
- always complete the full course.