Pathogens are microorganisms that cause infectious disease

Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by:

Direct contact - Some pathogens can be picked up by touching contaminated surfaces, including the skin (athlete’s foot is a fungus which makes skin itch and flake off. It’s more commonly spread by touching the same things as an infected person like shower floors and towels)

By water - Some pathogens can be picked up by drinking or bathing in dirty water (cholera is a bacterial infection that’s spread by drinking water contaminated with the diarrhoea of other sufferers)

By air - Pathogens can be carried in the air and can then be breathed in. Some airborne pathogens are carried in the air in droplets produced when you cough or sneeze (the influenza virus that causes flu is spread this way)

Bacteria and viruses may reproduce rapidly inside the body

Bacteria may produce poisons (toxins) that damage tissues and make us feel ill

Viruses live and reproduce inside cells, causing cell damage

Bacteria are very small cells that can reproduce rapidly inside your body. They can make you feel ill by producing toxins that damage your cells and tissues

Viruses are not cells. They’re smaller than bacteria. They live inside your cells and replicate themselves using the cells machinery to produce many copies of themselves. The cell will usually then burst, releasing all the new viruses. This cell damage is what makes you feel ill

Protists are all eukaryotes and most are single celled. Some are parasites. Parasites live on or inside other organisms and can cause them damage. They are often transferred to the organism by a vector which doesn’t get the disease itself (an insect that carries the protist)

Some fungi are single celled. Others have a body which is made up of hyphae which can grow and penetrate human skin and the surface of plants, causing diseases. The hyphae can produce spores which can be spread to other plants and animals

Reduce/prevent the spread of disease

Being hygienic - Using simple hygiene measures can prevent the spread of disease. For example, doing things like washing your hands thoroughly before preparing food or after you’ve sneezed can stop you infecting another person

Destroying vectors - By getting rid of the organisms that spread disease, you can prevent the disease from being passed on. Vectors that are insects can be killed using insecticides or by destroying their habitat so that they can no longer breed

Isolating infected individuals - If you isolate someone who has a communicable disease, it prevents them from passing it on to anyone else

Vaccination - Vaccinating people and animals against communicable diseases means that they can’t develop the infection and then pass it on to someone else.

Measles is a viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise. For this reason most young children are vaccinated against measles. The measles virus is spread by inhalation of droplets from sneezes and coughs

HIV initially causes flu like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells. Late stage HIV infection, or AIDS, occurs when the body’s immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of bodily fluids such as blood which occurs when drug users share needles

Tobacco Mosaic Virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive mosaic pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis

Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. In the UK, poultry are vaccinated against salmonella to control the spread. Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete

Gonorrhoea is an STD with symptoms of a thick yellow or green discharge and pain on urinating. It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared. Gonorrhoea is spread by sexual contact. The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom

Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth of the plant as photosynthesis is reduced. It is spread in the environment by water or wind. Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves

The pathogens that cause malaria are protists

The malarial protist has a life cycle that includes the mosquito. Malaria causes recurrent episodes of fever and can be fatal. The spread of malaria is controlled by preventing the vectors, mosquitos, from breeding and by using mosquito nets to avoid being bitten

Skin - Acts as a barrier to pathogens. It also secretes antimicrobial substances which kill pathogens

Nose - Hairs and mucus in the nose trap particles that could contain pathogens

Trachea and bronchi - Secrete mucus to trap pathogens . Lined with cilia which are hair like structures which waft the the mucus up to the back of the throat where it can be swallowed

Stomach - Produces hydrochloric acid which kills pathogens that make it that far from the mouth

If a pathogen enters the body the immune system tries to destroy the pathogen

White blood cells help to defend against pathogens by:

Phagocytosis - White blood cells can engulf foreign cells and digest them

Antibody production - Every invading pathogen has antigens on its surface. When some types of white blood cell come across a foreign antigen they will start to produce proteins called antibodies to lock onto the invading cells so that they can be found and destroyed by other white blood cells. The antibodies produced are specific to that type of antigen and they won’t lock on to any others. Antibodies are then produced rapidly and carried around the body to find all similar bacteria or viruses. If the person is infected with the same pathogen again the white blood cells will rapidly produce the antibodies to kill it and the person is naturally immune to that pathogen and won’t get ill

Antitoxin production - These counteract toxins produced by the invading bacteria

Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies. If the same pathogen re enters the body the white blood cells respond quickly to produce the correct antibodies preventing infection

Big outbreaks of disease can be prevented if a large percentage of the population is vaccinated. That way, even the people who aren’t vaccinated are unlikely to catch the disease because there are fewer people able to pass it on. If a large number of people aren’t vaccinated, the disease can spread quickly through them and lots of people will be ill at the same time

Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing ineffective bacteria inside the body. It is important that specific bacteria should be treated by specific antibiotics

The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. However the emergence of strains resistant to antibiotics is of great concern

Antibiotics cannot kill viral pathogens

Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens

It is difficult to develop drugs that kill viruses without also damaging the body’s tissues

Traditionally drugs were extracted from plants and microorganisms

The heart drug digitals originates from foxgloves

The painkiller aspirin originates from willow

Penicillin was discovered by Alexander Fleming from the Penicillium mould

Most new drugs are synthesised by chemists in the pharmaceutical industry. However the starting point may still be a chemical extracted from a plant

New medical drugs have to be tested and trialled before being used to check that they are safe and effective

New drugs are extensively tested for toxicity, efficacy and dose

Preclinical testing is done in a laboratory using cells tissues and live animals

Clinical trials use healthy volunteers and patients

Very low doses of the drug are given at the start of the clinical trial

If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug

In double blind trials, some patients are given a placebo

In preclinical testing, drugs are tested on human cells and tissues in the lab

However you can’t use human cells and tissues to test drugs that affect whole or multiple body systems (testing a drug for blood pressure must be done on a whole animal because it has an intact circulatory system)

The next step in preclinical testing is to test the drug on live animals. This is to test efficacy (whether the drug works and produces the effect you’re looking for), to find out about the toxicity (how harmful it is) and to find the best dosage (the concentration that should be given and how often)

The law in Britain states that any new drug must be tested on two different live mammals. Some people think it is cruel to test on animals but others believe this is the safest way to make sure a drug isn’t dangerous before it is given to humans

If a drug passes the tests on animals then it is tested on human volunteers in a clinical trial

First the drug is tested on healthy volunteers. This is to make sure that it doesn’t have any harmful side effects when the body is working normally. At the start of the trial, a very low dose of the drug is given and this is gradually increased

If the results of the tests on healthy volunteers are good, the drugs can be tested on people suffering from the illness. The optimum dose is found.

To test how well the drug works, patients are randomly put into two groups. One is given the new drug, the other is given a placebo. This is so the doctor can see the actual difference the drug makes and allows for the placebo effect (when the patient expects the treatment to work and so feels better even though the treatment isn’t doing anything)

Clinical trials are blind where the patient in the study doesn’t know whether they’re getting the drug or placebo. They are often double blind so that the doctors monitoring the patients and analysing the results aren’t subconsciously influenced by their knowledge

The results of drug testing and drug trials aren’t published until they have been through peer review which helps prevent false claims