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IGCSE Biology 0610 · Chapter 9 notes · ~14 min read

Diseases, immunity & drugs

What a pathogen is, how disease spreads, how your body fights back (skin, phagocytes, antibodies), how vaccines work, cholera, how we control disease and why antibiotic resistance matters.

PathogensTransmissionBody defencesAntibodiesVaccinationAntibioticsDrug misuse
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • A pathogen is an organism that causes disease. A transmissible disease is one where the pathogen passes from one host to another, directly or indirectly.
  • Your body has barriers that keep pathogens out (skin, mucus, stomach acid) and white cells that destroy those that get in. Phagocytes engulf them; lymphocytes make antibodies. EXT
  • Vaccines train your body to make memory cells without the illness. Antibiotics kill bacteria, never viruses, and overuse creates resistant bacteria.

By the end you can

  • Define pathogen and transmissible disease, and name the main groups of pathogen.
  • Describe direct and indirect transmission with examples.
  • Describe the body's defences: skin, nose hairs, mucus, stomach acid and white cells.
  • Describe how disease is controlled: clean water, hygienic food, personal hygiene, waste disposal and sewage treatment.
  • Describe the cause, transmission and effects of cholera.
  • State what a drug is and say how antibiotics work and why resistance matters.
  • Explain antigens, antibodies, memory cells and the difference between active and passive immunity. EXT
  • Explain how vaccination gives long-term protection. EXT
  • Explain how antibiotic resistance develops and how to slow it. EXT

1Pathogens and how they spread

What is a pathogen?

BACTERIAcholera, TB,pneumoniaVIRUSESinfluenza, HIV,measles, COVID-19FUNGIathlete’s foot,ringwormPROTOCTISTSmalaria(Plasmodium)PATHOGEN = an organism that causes diseaseTRANSMISSIBLE DISEASE = a disease whose pathogen passes between hosts
Four groups of organism can be pathogens. Know one disease for each.

Pathogen — an organism that causes disease.

Transmissible disease — a disease in which the pathogen can be passed from one host to another.

  • Not every disease is transmissible. Cancer, diabetes and scurvy are not caught from anyone.
  • Many pathogens are specific: the cholera bacterium makes you ill but not your cat.
Common mistake"Antibiotics cure flu." Flu is caused by a virus. Antibiotics only work on bacteria (see the last section).

Direct and indirect transmission

DIRECT TRANSMISSIONINDIRECT TRANSMISSIONpathogen passestouching, kissing, sexblood (shared needles,transfusions)mother to baby (placenta,breast milk)bites and scratchesobjectcontaminated food and watersurfaces (door handles)air: coughs and sneezes(droplets)vectors: mosquitoes, fliesDirect = the pathogen goes straight from one host to the next.Indirect = it travels on or in something in between (food, water, air, surface, animal).wash hands · cook food · clean water · cover coughs · control vectors
Direct: pathogen goes straight from host to host. Indirect: it travels on or in something in between.
  • Direct transmission: contact (touching, kissing), body fluids (blood, semen), and mother to baby across the placenta or in breast milk.
  • Indirect transmission: contaminated food and water, surfaces (door handles), the air (droplets from coughs and sneezes) and vectors: animals such as mosquitoes that carry the pathogen from host to host.
Memory trickDirect = Directly from person to person. Indirect = an Intermediate (object, food, water, air or animal) carries it.
Exam tipIn an exam, name what carries the pathogen ("contaminated water", "droplets in the air"). "By touching" alone is not enough for indirect.

2Body defences

Your first and second lines of defence

nose: hairs and mucustrap dust and microbeswindpipe: mucus and ciliatrap microbes and sweepthem up to be swallowedstomach: hydrochloricacid kills microbesskin: a tough,waterproof barrierblood: white cells (phagocytesand lymphocytes) destroypathogens that get in1st line: barriers that keep pathogens out · 2nd line: white cells that destroy those that get in
Barriers keep most pathogens out. White cells destroy the ones that get in.
DefenceHow it protects you
skina tough, waterproof barrier; a cut seals with a blood clot and scab
nose hairs and mucustrap dust and microbes before they reach the lungs
mucus and cilia in the windpipemucus traps microbes; cilia sweep it up to the throat to be swallowed
stomach acidhydrochloric acid kills most microbes in food and drink
white blood cellsdestroy pathogens that get inside the body (below)
Memory trickBarriers keep them out; white cells kill the ones that get in.

Phagocytes: the non-specific killers

1 CONTACT2 ENGULF3 TRAPPED4 DIGESTEDphagocyte meetsa pathogenit wraps roundthe pathogenthe pathogen isinside a vacuoleenzymes break itdownPhagocytosis: non-specific. Phagocytes engulf and digest ANY pathogen they meet.
A phagocyte flows around a pathogen and digests it. It does this to any pathogen it meets.
  • Phagocytosis = a phagocyte (a type of white blood cell) engulfs a pathogen and digests it with enzymes.
  • It is non-specific: the same phagocyte attacks any type of microbe it finds.
  • Phagocytes are found in the blood and can squeeze out of the capillaries into tissues to reach an infection (blood cells).
Common mistake"White cells eat microbes." Only phagocytes engulf. Lymphocytes do something different: they make antibodies.

3Immunity EXT

Antigens and antibodiesEXTENDED

ANTIGENS AND ANTIBODIESpathogen withANTIGENS(marker molecules)fits ✓antigen and antibodyshapes matchdoes not fit ✗a different antigen needsa different antibodyWHAT ANTIBODIES DO1 clump pathogens2 neutralise toxins3 mark for phagocytesLymphocytes make antibodies. Phagocytes do the engulfing. Do not mix them up.
EXT Each antigen has its own shape. An antibody fits only the antigen that matches.

Antigen — EXT a marker molecule on the surface of a cell or pathogen. Your body treats a foreign antigen as an invader.

Antibody — EXT a protein made by lymphocytes that binds to a specific antigen and helps destroy the pathogen.

  • EXT When a pathogen enters, a lymphocyte with a matching shape is activated and makes many antibodies. Antibodies clump pathogens together, neutralise toxins and mark pathogens so phagocytes find them easily.
  • EXT Antibodies are specific: one type fits one antigen only. That is why you need a separate vaccine for each disease.
Memory trickAntigen = ANTIbody GENerator: the thing that triggers antibody production.

Primary and secondary responseEXTENDED

0204060801000102030405060time (days)antibodies in the blood1st infection or vaccinere-infectionPRIMARY response:slow and smallSECONDARY response:fast and large(memory cells)
EXT The second time you meet the same pathogen, the response is much faster and bigger.
  • EXT First infection (primary response): it takes days to find the right lymphocyte and make enough antibodies. You may feel ill meanwhile. Some lymphocytes become memory cells that stay in the blood.
  • EXT Second infection (secondary response): memory cells recognise the antigen at once and make antibodies faster, in larger amounts. The pathogen is destroyed before you feel ill.
Exam tipDescribing a graph: say how fast (starts sooner, rises steeply), how high (peak is much higher) and how long (stays high longer).

Active and passive immunityEXTENDED

ACTIVE IMMUNITYyour OWN lymphocytes makethe antibodiesgained by infectionOR by vaccinationmemory cells are madeLONG-LASTINGtakes days to start workingPASSIVE IMMUNITYantibodies come READY-MADEfrom another personbreast milk, across the placenta,or an injection of antibodiesNO memory cells madeSHORT-TERMworks immediatelyActive = you make them. Passive = you are given them.
EXT Active: your own lymphocytes make the antibodies. Passive: antibodies are given to you ready-made.
FeatureActive immunityPassive immunity
who makes the antibodies?your own lymphocytesanother person (or animal)
how gainedinfection, or vaccinationbreast milk, across the placenta, or an injection of antibodies
memory cells?yesno
speedslow (days)immediate
how long?long-term (often years)short-term (antibodies break down)
  • EXT A newborn baby has passive immunity from the mother's antibodies (placenta and breast milk) while its own system develops.
Memory trickActive = Activated All by yourself. Passive = you're Protected by someone else's antibodies.

VaccinationEXTENDED

1vaccine givenweakened ordead pathogenor antigens2body respondsantibodies +memory cellsmade3memory keptmemory cellsstay in theblood for years4real infectionmemory cellsreact FAST: youdo not get illVaccination gives ACTIVE immunity without the illness.If most people are vaccinated, the pathogen has few hosts to spread to(this also protects the few who cannot be vaccinated).
EXT A vaccine gives the antigens without the disease, so memory cells are ready for the real infection.
  • EXT A vaccine contains a weakened or dead pathogen (or just its antigens). Your lymphocytes make antibodies and memory cells without you getting the disease.
  • EXT When the real pathogen arrives, memory cells make a fast, large secondary response, so you do not become ill.
  • EXT Vaccination = active immunity. It is long-term protection because memory cells stay in the blood.
  • EXT If most people are vaccinated, the pathogen has few hosts to spread to, so even people who cannot be vaccinated are better protected.
Common mistake"A vaccine is an injection of antibodies." No. A vaccine contains antigens (so you make your own antibodies = active). Injected antibodies are passive immunity.

4Cholera: a waterborne disease

Cause, spread and effects

1drink water contaminated withVibrio cholerae (a bacterium)2the bacteria multiply in the smallintestine3their toxin makes gut cellsrelease chloride ions into the gut4water follows by osmosis, out ofthe blood and cells5very watery diarrhoea: dehydrationand loss of saltsGUT LUMENBLOODred: chloride ions (Cl⁻) leaveblue: water follows by osmosisTREATMENToral rehydration: cleanwater + salts + glucosePREVENTIONclean drinking water,sewage treatment,good hygiene
Follow the five steps from contaminated water to dehydration.
  • Cause: the bacterium Vibrio cholerae. It is transmitted indirectly in water (and food) contaminated with faeces from an infected person.
  • Effects: the bacteria multiply in the small intestine and release a toxin. The gut loses large amounts of water, giving severe watery diarrhoea, dehydration and loss of salts. Without treatment this can be fatal.
  • EXT How the toxin works: it makes the gut lining cells release chloride ions into the gut. The gut contents now have a lower water potential than the blood, so water leaves the blood by osmosis into the gut (osmosis).
  • Treatment: oral rehydration (water, salts and glucose). Prevention: clean drinking water, sewage treatment and good hygiene.
Exam tipIn the exam link the toxin → gut → water loss → dehydration chain. Do not just write "it gives you diarrhoea".

5Controlling the spread of disease

Breaking the cycle

SEWAGERIVER / WATERFOOD & DRINKPEOPLEsewage treatmentcleans waste waterwater treatmentfilter and chlorinatehygienic foodcook, wash, coverpersonal hygienewash hands, safe wastebreak thecycleControl = barriers between the pathogen and the next host
Cholera spreads in a cycle. Each barrier cuts it.
  • Clean water supply: water is filtered to remove solids and chlorinated to kill microbes.
  • Sewage treatment: screens remove large objects, solids settle as sludge, bacteria break down the organic matter and the water is cleaned before it returns to rivers. This stops faeces contaminating drinking water.
  • Hygienic food preparation: wash hands and food, cook thoroughly, keep food covered and cold, keep raw and cooked food apart.
  • Personal hygiene: wash hands after the toilet and before eating; cover coughs and sneezes.
  • Waste disposal: rubbish and sewage are removed or buried. Uncollected waste breeds flies and rats, which are vectors.
Worked example

In severe cholera a patient can lose 1 litre of water per hour. How much is that in one day, and why is oral rehydration urgent?

  1. 1 day = 24 hours
  2. 1 × 24 = 24 litres per day
  3. an adult's body holds only about 40 litres of water, so this is more than half of it
  4. fast water and salt replacement is life-saving

6Drugs

What is a drug?

Drug — any substance taken into the body that modifies or affects chemical reactions in the body.

  • Drugs can be medicines (painkillers, antibiotics) or harmful substances. Whether a drug helps or harms depends on the drug, the dose and the person.

Antibiotics

1234a mutation makes a fewbacteria resistantantibiotic kills thenon-resistant bacteriaresistant survivorsreproducepopulation is nowmostly resistantThis is NATURAL SELECTION: the antibiotic is the selection pressure.To slow resistance: use antibiotics only when needed, finish the course,never use them for viruses (e.g. MRSA is resistant to many antibiotics).
Antibiotics select for resistant bacteria. This is natural selection in action.
  • Antibiotics (such as penicillin) kill bacteria or stop them growing. They do not work against viruses, because viruses live inside our cells and use our cell machinery. Do not take antibiotics for colds and flu.
  • Resistance: a mutation can make a bacterium resistant to an antibiotic. When the antibiotic kills the rest, the resistant survivors reproduce, so the whole population becomes resistant. The antibiotic no longer works.
  • EXT Resistance is natural selection: the antibiotic is the selection pressure, resistant bacteria survive and pass on the allele (natural selection).
  • EXT Slowing resistance: use antibiotics only when really needed, complete the whole course, and never for viral infections. MRSA is a bacterium resistant to many antibiotics, so infections are very hard to treat.
Worked example

A culture of 500 bacteria is treated with an antibiotic. 495 die. The survivors then divide every 30 minutes. Find the % resistant before, after the antibiotic, and the number after 3 hours.

  1. survivors = 500 − 495 = 5
  2. before: 5 ÷ 500 × 100 = 1% resistant
  3. after the antibiotic only resistant ones are left, so 100% resistant
  4. 3 hours = 180 min ÷ 30 = 6 divisions
  5. number = 5 × 26 = 5 × 64 = 320 resistant bacteria

Drug misuse: be aware

  • Not every drug taken is a medicine. Misuse harms the body and often causes addiction. This part is general knowledge; check whether your exam board asks for it.
SubstanceMain harm
tobacco smoketar causes lung cancer and damages cilia; carbon monoxide reduces the oxygen carried by blood; nicotine is addictive and raises heart rate
alcoholslows reactions and judgement (a depressant); long-term liver damage; addiction
heroin and other opiateshighly addictive; overdose can stop breathing; sharing needles spreads HIV and hepatitis
  • Worried about drugs or smoking? Speak to a doctor, parent or teacher. Help is available and early help works best.

7Test yourself

Test yourself — cover the answers first

Quick check

  1. Define pathogen.
  2. Define transmissible disease.
  3. Give one example of direct transmission.
  4. Give one example of indirect transmission.
  5. Name two ways the body stops pathogens entering.
  6. What is phagocytosis?
  7. Which bacterium causes cholera?
  8. How is cholera spread?
  9. What is a drug?
  10. Why do antibiotics not cure flu?
  11. EXT What is an antigen?
  12. EXT What cells make antibodies?
  13. EXT Why does a second infection make you less ill?
  14. EXT Is breast milk active or passive immunity?

Exam-style

  1. Explain how stomach acid protects the body from disease. [2]
  2. Explain why cholera causes dehydration. [3]
  3. Describe how clean water and sewage treatment reduce the spread of cholera. [3]
  4. EXT Explain how a vaccine protects you against a disease. [4]
  5. EXT Explain how antibiotic resistance develops in bacteria. [4]
  6. EXT Compare active and passive immunity. [3]
Show answers
  1. an organism that causes disease
  2. a disease in which the pathogen can be passed from one host to another
  3. touching or contact, body fluids, mother to baby
  4. contaminated food or water, droplets in the air, or a vector such as a mosquito
  5. skin, nose hairs and mucus, stomach acid
  6. a phagocyte engulfs a pathogen and digests it
  7. Vibrio cholerae
  8. by water or food contaminated with faeces
  9. a substance taken into the body that modifies or affects chemical reactions in the body
  10. flu is caused by a virus, and antibiotics only act on bacteria
  11. a marker molecule on the surface of a pathogen or cell that triggers antibody production
  12. lymphocytes
  13. memory cells make antibodies faster and in larger amounts
  14. passive
  15. the stomach contains hydrochloric acid
    which kills pathogens in food and drink
  16. the toxin makes the gut lose ions (chloride)
    water leaves the blood by osmosis into the gut
    causing watery diarrhoea and loss of water
  17. sewage treatment stops faeces contaminating water
    water is filtered and chlorinated
    so the bacteria cannot be swallowed in drinking water
  18. it contains weakened or dead pathogen or antigens
    lymphocytes make antibodies
    memory cells are made and stay in the blood
    on real infection antibodies are made faster and in larger amounts, so you do not become ill
  19. a mutation gives a bacterium resistance
    the antibiotic kills the non-resistant bacteria
    the resistant bacteria survive and reproduce
    so the population is mostly resistant
  20. active: your own lymphocytes make antibodies and memory cells are made; long-term
    passive: antibodies are given from another person, no memory cells, short-term
    active is slow to start, passive works immediately
Practise this chapter
Diseases & immunityDrugs
Next chapterGas exchange, respiration & excretion →
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