Diseases and immunity
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Pathogens and Transmissible Disease
- A pathogen is a disease-causing organism (e.g. bacteria, viruses, fungi, protoctists)
- A transmissible disease is a disease in which the pathogen can be passed from one host to another
Not all diseases are transmissible - for example, diabetes and lung cancer are not caused by pathogens, and cannot be passed between people.
Transmission of Pathogens
- Direct contact - a pathogen passes directly from one person to another through physical contact, blood or other body fluids (e.g. HIV, athlete's foot)
- Indirect - contaminated surfaces - a pathogen survives on an object or surface, and the next person touches it and then touches their face or mouth (e.g. the common cold virus on a door handle)
- Indirect - food - a pathogen in or on contaminated food is ingested (e.g. Salmonella in undercooked chicken)
- Indirect - animals - an animal carries a pathogen from one host to another without being affected itself (e.g. a mosquito transmitting the malaria parasite)
- Indirect - air - a pathogen in droplets expelled by coughing or sneezing is inhaled (e.g. influenza, tuberculosis)
- Indirect - contaminated water - a pathogen in water contaminated with sewage is ingested (e.g. cholera, typhoid)
Features of Viruses
The features of a virus are limited to:
- A protein coat surrounding the virus
- Genetic material (DNA or RNA) inside the protein coat
A virus consists of only a protein coat and genetic material - it has no cytoplasm, organelles or cell membrane
Common Mistakes
- ❌ Thinking viruses can be treated with antibiotics like bacteria
- ✓ bacteria are living organisms and can be killed by antibiotics. Viruses are not affected by antibiotics, and instead require antiviral drugs or prevention by vaccination
Body Defences Against Disease
- Skin - a physical barrier that prevents most pathogens from entering the body
- Hairs in the nose - trap dust, particles and pathogens from inhaled air, preventing them from entering the lungs
- Mucus - a sticky secretion lining the airways, nose and throat, which traps pathogens and particles
- Stomach acid - kills most pathogens swallowed with food or mucus, before they can infect the intestines
- White blood cells - defend against any pathogens that get past the physical barriers
Controlling the Spread of Disease
- Clean water supply - prevents waterborne pathogens (e.g. the cholera bacterium) from contaminating drinking water
- Hygienic food preparation - proper cooking kills pathogens, and keeping raw and cooked food separate and refrigerated prevents foodborne disease
- Good personal hygiene - regular handwashing removes pathogens from hands before they can be transferred to food or the mouth
- Waste disposal - proper disposal of rubbish prevents the build-up of materials that attract flies and rats, which can act as vectors
- Sewage treatment - removes pathogens from human waste before water is returned to the environment, preventing contamination of water supplies (the details of the treatment stages are not required)
Active Immunity (Supplement)
Active immunity is defence against a pathogen by antibody production in the body.
- Antibodies are proteins produced by white blood cells (lymphocytes), which act against a pathogen or the toxins it produces
- Active immunity is gained either after an infection by a pathogen, or by vaccination
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