Diseases and Immunity

The One Thing

Communicable diseases are caused by pathogens and spread between hosts through specific transmission routes. Infection can be prevented or limited by physical and chemical barriers, immune responses, vaccination, appropriate antibiotic use and public-health measures.

Reactions, Processes and Experiments

Key Terms

• Pathogen: A disease-causing organism or agent, including bacteria, viruses, fungi and protists.
• Communicable disease: A disease that can be transmitted from one person or organism to another.
• Bacterium: A single-celled microorganism that may cause disease by damaging cells or producing toxins.
• Virus: A non-cellular infectious particle that reproduces inside living cells and may destroy or damage them.
• Fungus: An organism that can cause infections, often affecting the skin or other body surfaces.
• Protist: A mostly single-celled organism; some protists cause diseases such as malaria.
• Transmission: The spread of a pathogen from one host to another.
• Direct contact transmission: Spread through physical contact, droplets from coughing or sneezing, or sexual contact.
• Indirect transmission: Spread through contaminated food, water, objects, blood or vectors.
• Vector: An organism that carries and transfers a pathogen between hosts, such as a mosquito.
• Incubation period: The time between infection by a pathogen and the appearance of symptoms.
• Physical barrier: A structure that prevents pathogens entering the body, such as intact skin.
• Chemical barrier: A substance that kills or inhibits pathogens, such as stomach hydrochloric acid or enzymes in tears.
• White blood cell: An immune cell that helps defend the body against pathogens.
• Phagocytosis: The process by which a white blood cell engulfs and digests a pathogen.
• Antigen: A molecule on the surface of a pathogen that is recognised as foreign by the immune system.
• Antibody: A protein produced by lymphocytes that binds specifically to a complementary antigen.
• Antitoxin: An antibody that binds to and neutralises toxins released by pathogens.
• Lymphocyte: A type of white blood cell that produces antibodies and contributes to immune memory.
• Antibiotic: A medicine that kills bacteria or prevents their reproduction.
• Antibiotic resistance: The ability of bacteria with resistance adaptations to survive treatment by an antibiotic.
• Vaccine: A preparation containing harmless pathogen material that stimulates immunity without causing the disease.
• Active immunity: Protection produced by the person's own immune system after infection or vaccination.
• Passive immunity: Temporary protection gained from antibodies made by another organism, such as antibodies passed from mother to baby.
• Immune memory: The ability of memory cells to respond faster and more strongly when the same pathogen enters the body again.
• Herd immunity: Protection of a population when a sufficiently large proportion is immune, reducing pathogen transmission.
• Malaria: A disease caused by a protist and transmitted by female Anopheles mosquitoes.
• Cholera: A bacterial disease usually spread through water contaminated with faeces; it causes severe diarrhoea and dehydration.
• Tuberculosis: A bacterial disease that mainly affects the lungs and can spread through droplets in the air.
• HIV/AIDS: HIV is a virus that attacks helper T cells; advanced infection can lead to AIDS and reduced immune function.

Easily Confused

• Direct transmission vs indirect transmission: Direct transmission occurs through physical contact, droplets or sexual contact; indirect transmission occurs through contaminated food, water, objects, blood or vectors.
• Physical barrier vs chemical barrier: Physical barriers block pathogen entry, whereas chemical barriers kill or inhibit pathogens.
• Antigen vs antibody: An antigen is a foreign molecule on a pathogen; an antibody is a protein produced by lymphocytes that binds to a complementary antigen.
• Antibody vs antitoxin: An antibody binds to a specific antigen, whereas an antitoxin specifically neutralises a toxin released by a pathogen.
• Active immunity vs passive immunity: Active immunity is produced by the person's own immune system after infection or vaccination; passive immunity comes from antibodies made by another organism and is temporary.
• Antibiotics vs vaccines: Antibiotics treat bacterial infections; vaccines stimulate immunity to prevent specific infections and do not normally cause the disease.
• Bacteria vs viruses: Bacteria are single-celled microorganisms that may be killed by antibiotics; viruses reproduce inside living cells and are not treated by antibiotics.
• Malaria vs cholera: Malaria is caused by a protist and transmitted by female Anopheles mosquitoes; cholera is bacterial and usually spread through water contaminated with faeces.
• HIV transmission vs casual contact: HIV can spread through infected blood, semen, vaginal fluids and breast milk, but not through sharing utensils or hugging.
• Antibiotic resistance vs antibiotic treatment: Antibiotic treatment kills susceptible bacteria, whereas antibiotic resistance allows some bacteria to survive and reproduce during treatment.

What Gets Asked

• Identifying transmission routes: Questions may ask how malaria, cholera, tuberculosis or HIV is transmitted. Marks are lost by confusing mosquito-vector transmission, contaminated-water transmission, airborne droplets and transmission through infected body fluids.
• Explaining body defences: Questions may require the roles of skin, blood clotting, mucus, cilia, tears, saliva and stomach acid. Marks are lost by treating all barriers as immune cells or by failing to distinguish physical from chemical barriers.
• Describing the immune response: Questions may ask how phagocytes, lymphocytes, antibodies, antitoxins and memory cells defend the body. Marks are lost by stating that antibodies engulf pathogens, rather than assigning engulfment to phagocytes.
• Explaining vaccination and immunity: Questions may require the contents of a vaccine, antibody production, memory-cell formation, the secondary immune response, active immunity or herd immunity. Marks are lost by describing vaccination as treatment with antibiotics or by confusing active and passive immunity.
• Explaining antibiotic resistance: Questions may ask how resistance develops or how it is increased. Marks are lost by stating that bacteria deliberately adapt, rather than explaining that random genetic variation is followed by survival and reproduction of resistant bacteria.
• Evaluating disease-control measures: Questions may compare hygiene, sanitation, vaccination, vector control, antibiotic use and public-health programmes. Marks are lost by applying one control method to every disease, despite different pathogens requiring different measures.