Infectious diseases

• Name the pathogen and its type; trace the chain; put each measure on the link it breaks and say what limits it. Penicillin stops cross-links being made — it never breaks the wall down.
• Two bacterial diseases, one protoctist disease, one viral disease — seven named pathogens in all.
• The mosquito is the vector, not the cause. Penicillin stops cross-links being made in growing bacteria; water enters; osmotic lysis. Every measure breaks a link, and every measure has a limit.

• 10.1.1 I can state — state that infectious diseases are caused by pathogens and are transmissible
• 10.1.2 I can state — state the name and type of pathogen that causes each of the following diseases: cholera – caused by the bacterium Vibrio cholerae; malaria – caused by the protoctists Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale and Plasmodium vivax; tuberculosis (TB) – caused by the bacteria Mycobacterium tuberculosis and Mycobacterium bovis; HIV/AIDS – caused by the human immunodeficiency virus (HIV)
• 10.1.3 I can explain — explain how cholera, malaria, TB and HIV are transmitted
• 10.1.4 I can discuss — discuss the biological, social and economic factors that need to be considered in the prevention and control of cholera, malaria, TB and HIV (details of the life cycle of the malarial parasite are not expected)
• 10.2.1 I can outline — outline how penicillin acts on bacteria and why antibiotics do not affect viruses
• 10.2.2 I can discuss — discuss the consequences of antibiotic resistance and the steps that can be taken to reduce its impact

Conclusion. Resistance becomes more common through the use of antibiotics itself, so no single step is enough. Its impact is reduced most by using fewer antibiotics and using them correctly, by preventing infections, and by developing new drugs, and that needs doctors, patients, farmers, drug companies and governments acting together.

• “Mosquitoes give you malaria, so the mosquito is the pathogen.” Correct Malaria is caused by protoctists, four species of Plasmodium. The female Anopheles mosquito is the vector: it carries the parasite from one person’s blood to the next. A vector is not a pathogen.
• “TB is caused by a virus” / “malaria is caused by a bacterium.” Correct Two bacterial diseases (cholera, TB), one caused by protoctists (malaria), one by a virus (HIV/AIDS). A question that asks for the type of pathogen wants that word.
• “M. bovis is breathed in from someone who is coughing.” Correct M. tuberculosis travels in airborne droplets. M. bovis reaches people mainly from cattle, in unpasteurised milk and undercooked meat.
• “HIV and AIDS are the same thing.” Correct HIV is the virus. AIDS (acquired immune deficiency syndrome) is the late stage of the disease that HIV infection causes.
• “Penicillin breaks down the bacterial cell wall.” Correct Penicillin inhibits the enzymes that form cross-links in peptidoglycan. The wall already there is untouched; new wall made by a growing bacterium is weak, water enters by osmosis, and the cell bursts (osmotic lysis).
• “Antibiotics do not work on viruses because viruses are too small.” Correct Size is not the reason. A virus has no cell wall, no ribosomes and no metabolism of its own, and it replicates inside host cells using the host’s machinery, so an antibiotic has nothing to act on.
• “The bacteria have developed an immunity to the antibiotic.” Correct Bacteria are resistant: they carry a gene that lets them survive the antibiotic. Immunity belongs to an animal’s immune system.
• A discuss answer written as a list of measures. Correct Each measure needs how it breaks transmission and what limits it, labelled biological, social or economic. Then weigh the measures in a conclusion.
• “The Anopheles mosquito is the pathogen that causes malaria.” Repair The pathogen is the protoctist Plasmodium; the female Anopheles is the vector that carries it between people.
• “Malaria is a bacterial disease.” Repair Malaria is caused by protoctists: Plasmodium falciparum, P. malariae, P. ovale and P. vivax.
• “TB is a viral infection of the lungs.” Repair TB is caused by the bacteria Mycobacterium tuberculosis and Mycobacterium bovis.
• “HIV and AIDS mean the same thing.” Repair HIV is the virus; AIDS is the late stage of the disease it causes.
• “Cholera spreads through the air in crowded places.” Repair Cholera is faecal–oral: water or food contaminated with faeces. Crowding matters because many people share one contaminated source.
• “You can catch HIV from a mosquito bite or a shared cup.” Repair HIV passes only in sexual fluids, in blood, and from mother to child.
• “M. bovis spreads in droplets when an infected person coughs.” Repair M. bovis reaches people mainly from cattle, in unpasteurised milk and undercooked meat.
• “Oral rehydration therapy cures cholera by killing the bacteria.” Repair ORT replaces the water and ions lost in diarrhoea, which prevents death from dehydration. It does not remove the bacteria.
• “Penicillin dissolves the cell wall of bacteria.” Repair It inhibits the enzymes that form cross-links; the new wall is weak, water enters by osmosis, and the cell bursts.
• “Penicillin kills every bacterium on the plate.” Repair It acts on bacteria that are growing and making new wall, and only on those it reaches in a high enough concentration.
• “Viruses are too small for antibiotics to hit.” Repair A virus has no wall, no ribosomes and no metabolism, and it replicates inside host cells, so there is no target.
• “Taking antibiotics causes bacteria to change their DNA so that they resist the drug.” Repair The antibiotic does not create the resistance gene. Resistant bacteria already carry a gene that lets them survive the antibiotic.
• “Resistant bacteria have built up an immunity.” Repair Use resistant. Immunity is a property of an animal’s immune system.
• A discuss answer on TB: “BCG, antibiotics, better housing, pasteurisation.” Repair Give each measure with how it works and what limits it, say which kind of factor, then weigh them. A list of names is not a discussion.
• “The measurements of the zones might have been inaccurate.” Repair Name the source and the improvement: the zone edge is blurred, a random error; read against a dark background lit from below, and average four diameters.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. The four that carry topic 10’s outcomes, in the syllabus’s own words: state — “express in clear terms”; explain — “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”; outline — “set out the main points”; discuss — “write about issue(s) or topic(s) in depth in a structured way”. Questions on this topic also use describe (“state the points of a topic / give characteristics and main features”) and suggest (“apply knowledge and understanding to situations where there are a range of valid responses to make proposals / put forward considerations”). Check any other word against the syllabus’s table.
• Interleave with the chapters that use this one. When you reach topic 11, re-answer “why is there no vaccine for HIV?” with the immune system in full (11.1.3, 11.2.5). When you reach topic 17, re-answer worked example 4 with the mechanism by which resistance becomes common. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 10 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A Paper 1 item on this topic turns on one precise fact: the type of a named pathogen, the route of a named disease, the difference between a vector and a pathogen, or what penicillin does to a growing bacterium. The wrong options are the near-misses in the danger zones below. In Paper 2 you state names, explain a route or penicillin’s action, outline why antibiotics do not affect viruses, and discuss control or resistance at length, measure by measure.
• Questions give you data to interpret: cases per 10 000 people before and after a measure, the percentage of samples resistant to an antibiotic by year, or the diameters of clear zones round antibiotic discs. The mathematics is AS: percentage change from the initial value, and the area of a circle (\(\pi r^2\)) for a clear zone. You recall both; neither is printed. Every count in this chapter is a fictional learning value.
• The syllabus names no practical for topic 10. The closest investigation is the effect of antibiotic concentration on bacterial growth: concentration by serial dilution (independent variable), the mean clear-zone diameter from two diameters at right angles (dependent variable), a sterile-water disc as the control, aseptic technique throughout, and the blurred zone edge as the main source of error.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. The four that carry topic 10’s outcomes, in the syllabus’s own words: state — “express in clear terms”; explain — “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”; outline — “set out the main points”; discuss — “write about issue(s) or topic(s) in depth in a structured way”. Questions on this topic also use describe (“state the points of a topic / give characteristics and main features”) and suggest (“apply knowledge and understanding to situations where there are a range of valid responses to make proposals / put forward considerations”). Check any other word against the syllabus’s table.