BIOLOGY · Stage 7 · Chapter 3: Microorganisms and Decomposers
Biology 3.3 Microorganisms and disease — Pathogens and Infectious Diseases
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Lesson summary
3.3 Microorganisms and disease — Pathogens and Infectious Diseases — Alhussan International Academy — Biology (Stages 7–9) notes: Explain what pathogens are, how they spread and how Pasteur's anthrax experiment supported the germ theory of disease.Pathogen = a microorganism that …
- Infectious (communicable) disease
- How pathogens harm
- Different action
- Droplets in the air
- Direct contact
- Contaminated food and water
Section 1 of 7Pathogens and infectious disease
- Pathogen = a microorganism that causes disease.
- Pathogens can be bacteria, viruses, fungi or protoctists.
- Most microorganisms are harmless or helpful.
- Infectious (communicable) disease = a disease that can be passed from one person or animal to another.
- How pathogens harm
- viruses destroy the cells they invade, and many bacteria release poisons called toxins.
- Different action
- bacteria multiply outside body cells, while viruses must enter cells and use them to copy themselves, so treatments differ.
Section 2 of 7Examples of diseases
| Type of pathogen | Diseases | Notes |
|---|---|---|
| Bacteria | Cholera, tuberculosis, food poisoning (Salmonella), anthrax | Treated with antibiotics |
| Virus | Influenza, measles, HIV, COVID-19 | Antibiotics do not work; vaccines help |
| Fungus | Athlete's foot, ringworm | Treated with antifungal creams |
| Protoctist | Malaria (Plasmodium) | Carried by mosquitoes |
Section 3 of 7How pathogens spread
- Droplets in the air
- coughs and sneezes release tiny droplets containing pathogens, as in influenza and colds.
- Direct contact
- touching an infected person or surface spreads athlete's foot and ringworm.
- Contaminated food and water
- eating or drinking pathogens spreads cholera and Salmonella.
- Body fluids
- pathogens pass through blood or other fluids, as with HIV.
- Vectors
- an animal carries the pathogen from one host to another.
- The female Anopheles mosquito carries the malaria parasite in its saliva and passes it on when it bites.
Section 4 of 7Preventing the spread of disease
- Hand washing
- it removes pathogens from the skin before they reach the mouth.
- Cooking food thoroughly
- it kills bacteria such as Salmonella.
- Covering the nose and mouth when sneezing
- it reduces droplets.
- Staying at home when infected
- it limits contact.
- Clean water and sewage treatment
- they stop cholera, which spreads when sewage contaminates water supplies.
- Malaria control
- kill mosquitoes, drain standing water and use bed nets.
- Tracing contacts
- health services track outbreaks, find contacts of infected people and advise them to isolate.
Section 5 of 7The body's defences
- Barriers
- skin, mucus and cilia in the airways, and acid in the stomach.
- White blood cells
- they destroy pathogens that get past the barriers.
- They can engulf pathogens or make antibodies, which stick to the pathogen and mark it for destruction.
- Symptoms
- signs such as fever show the body is fighting back.
- Memory cells
- they remain after the infection, so a second infection by the same pathogen is dealt with faster.
- Incubation period = the time between catching a pathogen and showing symptoms.
- People can spread some diseases during this time without knowing they are ill.
Section 6 of 7Louis Pasteur and the germ theory
- Background
- in the 19th century nobody knew the cause of infectious diseases that killed many people and animals.
- Ideas ranged from bad smells in the air to curses.
- Pasteur
- a French scientist determined to find the cause of infections after three of his children died of infectious disease.
- Germ theory
- diseases are caused by germs, an old word for microorganisms, passed from one person or animal to another.
- Hypothesis
- anthrax, a deadly disease of people and farm animals, is caused by a germ passed from one animal or person to another.
- Idea
- grow anthrax germs, weaken them and inject them into sheep as a vaccine.
- Problems
- He struggled to grow anthrax germs in his laboratory and found it hard to make a vaccine from them.
- His rival Jean-Joseph Toussaint had already made a vaccine by a different method, without proving it worked.
- The vet Hippolyte Rossignol did not believe germs caused disease and challenged Pasteur to a public test.
Section 7 of 7Pasteur's anthrax investigation
- On 5 and 17 May 1881 Pasteur gave 24 healthy sheep, 6 cows and 1 goat his anthrax vaccine, and a similar group of healthy animals was not vaccinated.
- On 31 May all the animals were infected with live anthrax from diseased animals.
- Prediction: the vaccinated animals would survive and the unvaccinated animals would develop anthrax and die.
- Observation on 2 June 1881: all the vaccinated animals were alive and healthy, and all the unvaccinated animals were dead or dying.
- Conclusion: the germ theory of disease was correct and the vaccine protected the animals.
- Impact
- the challenge was very public, so many scientists saw the result themselves.
- Pasteur convinced scientists that infectious diseases are caused by microorganisms.
- Infectious diseases can be prevented or cured by attacking those microorganisms.
- Scientific method
- hypotheses are supported or contradicted by evidence, and outcomes can be predicted from scientific understanding.
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