Pathogens cause communicable disease. This topic covers what they are, how the body resists them, and how medicines are found and tested.
A pathogen is a disease-causing microorganism or virus: bacteria, viruses, protists or fungi. Bacteria can cause disease by multiplying in the body, and may produce toxins that damage tissues. Viruses reproduce inside cells and damage them.
Pathogens spread by air, in droplets from coughs and sneezes; by direct contact, including sexual contact and touching contaminated surfaces; and by water or food. Spread is reduced by hygiene, isolating infected individuals, vaccination, and destroying vectors.
Measles spreads in droplets and causes a fever and a red skin rash. It can cause serious complications and can sometimes be fatal. Vaccination greatly reduces the risk of infection and of serious disease.
HIV spreads by sexual contact or by exchange of body fluids such as sharing needles. It causes flu-like symptoms at first, then attacks the immune cells; when the immune system is badly damaged the condition is called AIDS. Antiretroviral drugs slow its progress.
Tobacco mosaic virus affects many plants, giving a distinctive mosaic pattern of discolouration on the leaves and reducing photosynthesis and growth.
Salmonella food poisoning is spread by food that is undercooked or prepared in unhygienic conditions; the bacteria produce toxins causing fever, cramps, vomiting and diarrhoea. In the UK poultry are vaccinated against it.
Gonorrhoea is a sexually transmitted disease giving a thick yellow or green discharge and pain on urinating. It was treated with penicillin until resistant strains appeared; it is now controlled with a specific antibiotic and by using barrier contraception.
Rose black spot is a fungal disease that produces purple or black spots on rose leaves, which then turn yellow and drop. Photosynthesis is reduced and growth suffers. It spreads in water and by wind, and is treated with fungicides and by removing and destroying affected leaves.
Malaria is caused by a protist and spread by mosquitoes, which act as vectors. It causes recurrent fevers and can be fatal. It is controlled by stopping the vectors breeding and by mosquito nets.
The first line of defence is non-specific: the skin is a barrier and produces antimicrobial secretions; the nose has hairs and mucus; the trachea and bronchi are lined with mucus and cilia that sweep it away; and the stomach produces hydrochloric acid that kills swallowed pathogens.
If a pathogen gets past those, white blood cells respond in three ways: phagocytosis, engulfing and digesting the pathogen; producing antibodies, which are specific to that pathogen’s antigens; and producing antitoxins that neutralise its toxins.
A vaccine exposes the immune system to antigens from a pathogen, using weakened or inactive pathogens or parts of them. The white blood cells respond by producing antibodies, and afterwards memory cells remain. If the pathogen is encountered later, antibodies are produced far more quickly and in greater quantity, so the immune response is much faster and the chance of becoming seriously ill is greatly reduced.
If a large proportion of the population is vaccinated, the pathogen cannot spread easily even among those who are not. This is herd immunity. This helps protect people who cannot be vaccinated or who do not respond well to vaccination, including some immunocompromised people and infants who are too young for a particular vaccine.
Antibiotics, such as penicillin, kill bacteria inside the body or stop them growing and reproducing. Specific bacteria need specific antibiotics, and their use has greatly reduced deaths from bacterial disease.
They do not affect viruses, because viruses live and reproduce inside the body’s own cells, which makes them hard to attack without damaging the cells too. Antivirals inhibit viral replication rather than killing the virus outright.
Painkillers such as aspirin treat the symptoms only. They do not kill the pathogen, so the infection is unaffected.
Antibiotic resistance is increasing because resistant bacteria survive treatment and reproduce, and their offspring are resistant too. It is slowed by not prescribing antibiotics for viral infections or trivial complaints, and by patients taking antibiotics as prescribed and completing the course they are given.
Traditionally drugs came from plants and microorganisms: digitalis from foxgloves, aspirin from willow, and penicillin discovered by Alexander Fleming from Penicillium mould. Most new drugs are now synthesised by chemists, though the starting point may still be a natural compound.
Testing has stages. Preclinical testing uses cells, tissues and live animals to check toxicity, efficacy and dose. Clinical trials often begin with small numbers of healthy volunteers at very low doses to test safety, followed by patients to find the optimum dose, although some drugs are first tested in patients.
Trials are usually randomised and double blind: participants are allocated to treatment or placebo groups, and neither the participants nor the researchers assessing them know which they received until the study is complete, so expectation cannot influence the result. Findings are then peer reviewed before publication.
Monoclonal antibodies are identical antibodies produced from a single clone of cells, so they bind to one specific antigen.
They are made by stimulating a mouse lymphocyte to produce a particular antibody, then fusing it with a tumour cell to give a hybridoma that both makes the antibody and divides indefinitely.
They are used in pregnancy tests, to measure hormone levels, to identify particular molecules in a sample, and to deliver drugs to cancer cells while sparing healthy ones. They can cause side effects, so treatments must be carefully tested and monitored.
Plants have physical defences, including a waxy cuticle, cell walls and layers of dead cells such as bark. Chemical defences include antibacterial compounds and poisons. Mechanical defences include thorns, hairs, leaves that droop or curl on contact, and mimicry.
Disease can be detected from stunted growth, spots on leaves, areas of decay, growths, malformed stems or leaves, and discolouration. It can be identified by reference to a gardening manual, by taking the plant to a laboratory, or with a testing kit using monoclonal antibodies.
This is a simplified model, described here for bacteriophages, which are viruses that infect bacteria. Not all viruses behave in this way. In the lytic pathway the bacteriophage attaches to a host cell and injects its genetic material. The cell’s machinery is used to make new viruses, which assemble and burst the cell open, killing it and releasing them.
Some bacteriophages can instead enter a lysogenic state, in which the injected genetic material is inserted into the host's DNA and lies dormant, copied each time the cell divides. A trigger can later cause it to become active and enter the lytic pathway.
