Revision Library

revisionlibrary.co.uk

GCSE · Biology

B3a Communicable Diseases and Pathogens

AQA 8464 · Calculator allowed · about 110 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

A communicable disease is caused by a pathogen and can spread from one organism to another.

(a)Define the term 'pathogen'.(1)

(b)Define the term 'communicable disease'.(1)

(c)Name the four types of pathogen that can cause communicable disease.(4)

(Total for Question 1 is 6 marks)

Different communicable diseases are caused by different types of pathogen.

(a)State the type of pathogen (bacterium, virus, fungus or protist) that causes measles.(1)

(b)State the type of pathogen (bacterium, virus, fungus or protist) that causes salmonella food poisoning.(1)

(c)State the type of pathogen (bacterium, virus, fungus or protist) that causes rose black spot.(1)

(d)State the type of pathogen (bacterium, virus, fungus or protist) that causes malaria.(1)

(Total for Question 2 is 4 marks)

Tobacco mosaic virus (TMV) is a viral pathogen that infects tobacco plants and many other plant species, including tomatoes.

(a)Give two symptoms shown by a plant infected with TMV.(2)

(b)Explain how the symptoms of TMV reduce the rate of photosynthesis in an infected leaf.(2)

(c)TMV is described as having a wide host range, meaning it can infect many different species of plant. Suggest why this makes TMV difficult to control on a farm.(2)

(Total for Question 3 is 6 marks)

Measles is a communicable disease caused by a virus. In the UK, most children are offered the MMR vaccine, which protects against measles.

(a)State the type of pathogen that causes measles.(1)

(b)Give two symptoms of measles.(2)

(c)Measles is spread by inhalation of droplets released into the air from the coughs and sneezes of an infected person. Explain why measles can spread quickly through a primary school.(2)

(d)Explain how vaccinating most of the children in a school against measles can also help to protect the small number of children who have not been vaccinated.(3)

(Total for Question 4 is 8 marks)

HIV (human immunodeficiency virus) initially causes a flu-like illness. If untreated, HIV can severely damage the body's ability to fight off other infections and diseases, a condition known as AIDS.

(a)Give two ways that HIV can be transmitted between people.(2)

(b)Suggest two ways that the transmission of HIV between people could be reduced.(2)

(c)HIV can only be transmitted by direct contact with body fluids, whereas measles is transmitted by droplet infection through the air. Suggest why this difference affects how easily each disease can spread through a population.(3)

(Total for Question 5 is 7 marks)

Salmonella is a bacterium that can cause food poisoning in humans. In the UK, poultry (chickens) are routinely vaccinated against Salmonella.

(a)State the type of pathogen that causes salmonella food poisoning.(1)

(b)Give two symptoms of salmonella food poisoning.(2)

(c)Explain how vaccinating poultry against Salmonella reduces the number of cases of salmonella food poisoning in humans.(3)

(d)State two things a person preparing chicken at home can do to reduce the risk of salmonella food poisoning.(2)

(Total for Question 6 is 8 marks)

A food hygiene inspector used aseptic technique to swab a chopping board that had been used to prepare raw chicken. The swab was used to inoculate a sterile agar plate, which was then incubated at 25 degrees C.

(a)Give two aseptic technique steps the inspector should use when inoculating the agar plate, to prevent unwanted microorganisms from contaminating it.(2)

(b)The population of bacteria on the plate doubled every 20 minutes. If there were 4 bacteria present on the plate at the start of incubation (time = 0), calculate the number of bacteria present after 2 hours of incubation. Show your working.(3)

(c)Suggest one reason why the value calculated in part (b) may be an overestimate of the number of bacteria that would actually be found on a real chopping board after 2 hours.(1)

(d)Suggest why school laboratories usually incubate plates like this at about 25 degrees C rather than at 37 degrees C (body temperature).(2)

(e)Predict, with a reason, how the number of colonies on a plate from the same swab, but incubated at 4 degrees C (a fridge), would compare with the plate incubated at 25 degrees C.(2)

(Total for Question 7 is 10 marks)

Gonorrhoea is a sexually transmitted infection (STI). In the past, it was treated effectively with the antibiotic penicillin.

(a)State the type of pathogen that causes gonorrhoea.(1)

(b)State how gonorrhoea is transmitted between people.(1)

(c)Give two symptoms of gonorrhoea.(2)

(d)Explain why penicillin is now less effective at treating some gonorrhoea infections than it used to be.(2)

(Total for Question 8 is 6 marks)

Malaria is a disease caused by a protist, Plasmodium, and is transmitted between people by a vector: the female Anopheles mosquito.

(a)Name the type of pathogen that causes malaria.(1)

(b)Name the vector that spreads malaria between humans.(1)

(c)Explain how the malaria parasite is transmitted from an infected person to an uninfected person by the mosquito vector.(3)

(d)A region recorded 24500 cases of malaria in one year. After insecticide-treated bed nets were distributed to 80% of households, the number of cases fell to 6125 in the following year. Calculate the percentage decrease in the number of malaria cases.(3)

(e)Suggest two methods, other than bed nets, that could be used to reduce the spread of malaria in this region.(2)

(Total for Question 9 is 10 marks)

Rose black spot is a fungal disease that affects the leaves of rose plants.

(a)State the type of pathogen that causes rose black spot.(1)

(b)Describe two symptoms of rose black spot on a rose plant.(2)

(c)Explain how the loss of leaves caused by rose black spot can reduce a rose plant's growth.(2)

(d)Give two ways a gardener could try to control the spread of rose black spot.(2)

(Total for Question 10 is 7 marks)

Before treating a plant, a gardener or farmer needs to identify whether it has a communicable disease.

(a)Give two visible signs that could suggest a plant is diseased.(2)

(b)Give two ways, other than sending a sample away to a laboratory, that a gardener could find out what is causing a disease in a plant.(2)

(c)A field of wheat plants shows patches of stunted, discoloured plants. Explain how looking at the distribution pattern of the affected plants across the field could help a farmer work out whether the cause is a communicable pathogen or a mineral ion deficiency in the soil.(3)

(Total for Question 11 is 7 marks)

Plants have physical, mechanical and chemical defence responses that help protect them against pathogens and herbivores.

(a)Give one example of a physical plant defence and one example of a mechanical plant defence.(2)

(b)Explain how a waxy cuticle acts as a physical barrier to help prevent pathogens infecting a leaf.(2)

(c)Some plants produce chemicals that are poisonous to insects that try to feed on their leaves. Suggest how this reduces the plant's risk of catching a communicable disease.(2)

(Total for Question 12 is 6 marks)

A crop researcher sampled 250 wheat plants in a large field and found that 35 of them showed symptoms of a fungal leaf disease.

(a)Calculate the percentage of sampled plants showing symptoms of the disease.(2)

(b)The field contains 12000 wheat plants in total. Assuming the same percentage of plants is infected across the whole field, calculate the total number of infected plants in the field.(2)

(c)The researcher could recommend removing (roguing) all infected plants, or treating the whole field with a fungicide. Suggest one advantage and one disadvantage of removing the infected plants instead of treating the whole field with fungicide.(2)

(Total for Question 13 is 6 marks)

Malaria is a communicable disease caused by a protist and spread between people by a mosquito vector. Measles is a communicable disease caused by a virus and spread between people by droplet infection from coughs and sneezes.

Compare the transmission and prevention of malaria and measles. In your answer, evaluate which of the two diseases would be more difficult for a country with limited money for healthcare to control, using the methods available for reducing the transmission of each disease.

(Total for Question 14 is 6 marks)

A glasshouse contains 500 tomato plants. On day 0, 8 plants were found to be infected with a bacterial plant disease. Assuming no control measures were introduced, the number of infected plants doubled every 3 days.

(a)Calculate the number of infected plants there would have been by day 12, if this pattern of doubling had continued. Show your working.(3)

(b)On day 12, growers introduced control measures (removing infected plants and increasing ventilation), and the rate of new infections fell to a constant 5 newly infected plants per day for the following 10 days. Calculate the total number of plants that had become infected in the glasshouse by day 22 (including those infected before day 12).(2)

(c)The disease is spread by water splash between neighbouring plants during overhead irrigation. Suggest two changes to the glasshouse's irrigation or growing practices that could reduce further transmission of this disease.(2)

(Total for Question 15 is 7 marks)

A student used a light microscope to examine a prepared slide of leaf tissue from a rose plant showing symptoms of rose black spot, as part of an investigation to confirm the cause of the disease.

(a)The microscope was fitted with a x10 eyepiece lens and a x40 objective lens. Calculate the total magnification used to view the slide.(1)

(b)The student made a scale drawing of a fungal hypha seen on the slide, using a total magnification of x300. In the drawing, the hypha measured 15 mm long. Calculate the actual length of the hypha, in micrometres (um). Show your working.(3)

(c)Suggest one reason why a farmer might want to use microscopy to confirm rose black spot, rather than relying on the plant's visible symptoms alone.(2)

(d)State one technique the student should use when preparing the leaf sample slide, to avoid damaging the sample.(1)

(Total for Question 16 is 7 marks)

Mark scheme · B3a Communicable Diseases and Pathogens

Question 1

(a) B1 a microorganism that causes disease, oe
(a) Answer: A microorganism that causes disease.
(b) B1 a disease caused by a pathogen that can be passed (spread) from one organism/person to another, oe
(b) Answer: A disease caused by a pathogen that can be spread from one organism (or person) to another.
(c) B1 bacteria (bacterium)
(c) B1 viruses (virus)
(c) B1 fungi (fungus)
(c) B1 protists (protist)
(c) Answer: Bacteria, viruses, fungi and protists.

Question 2

(a) B1 virus cao
(a) Answer: Virus.
(b) B1 bacterium cao
(b) Answer: Bacterium.
(c) B1 fungus cao
(c) Answer: Fungus.
(d) B1 protist cao
(d) Answer: Protist.

Question 3

(a) B1 a mosaic pattern of discolouration (light green/yellow and dark green patches) appears on the leaves, oe
(a) B1 the plant's growth is stunted, oe
(a) Answer: A mosaic pattern of light and dark green (or yellow) patches on the leaves, and stunted growth.
(b) B1 the discoloured (mosaic) patches on the leaf contain less chlorophyll, so less light can be absorbed, oe
(b) B1 this reduces the rate of photosynthesis, so less glucose is produced for growth, oe
(b) Answer: The discoloured patches contain less chlorophyll, so less light is absorbed. This reduces the rate of photosynthesis and the amount of glucose produced for growth.
(c) B1 the virus can survive/be carried on many other plant species (including weeds), not just the main crop, oe
(c) B1 so removing or treating only the main crop does not remove the virus from the field/farm, and it can reinfect new crops from these other host plants, oe
(c) Answer: Because TMV can infect many other plant species (including weeds), it can persist in the field even if the main crop is removed or treated, and can reinfect new crops from these other host plants.

Question 4

(a) B1 virus cao
(a) Answer: Virus.
(b) B1 a red skin rash, oe
(b) B1 a fever (high temperature), oe (accept: sore, red eyes)
(b) Answer: A red skin rash and a fever (high temperature).
(c) B1 children in a primary school classroom are in close contact with each other for long periods of time, oe
(c) B1 so droplets from an infected child's coughs/sneezes are easily inhaled by many other children nearby, oe
(c) Answer: Children in a primary school are in close contact for long periods, so droplets from an infected child's coughs and sneezes are easily breathed in by many other children.
(d) B1 vaccinating most of the population means fewer people can become infected with (and pass on) the measles virus, oe
(d) B1 this reduces the overall number of measles cases circulating within the school/population, oe
(d) B1 so unvaccinated children are much less likely to come into contact with an infected person, reducing their chance of catching measles, oe
(d) Answer: Vaccinating most children means fewer people can become infected and pass the virus on, so there are fewer cases circulating overall. This makes it much less likely that an unvaccinated child will come into contact with an infected person.

Question 5

(a) B1 sexual contact (unprotected sex) with an infected person, oe
(a) B1 exchange of bodily fluids, e.g. sharing needles contaminated with infected blood, or from mother to baby during pregnancy/birth/breastfeeding, oe
(a) Answer: Any two of: unprotected sexual contact; sharing infected needles; exchange of bodily fluids; from mother to baby during pregnancy, birth or breastfeeding.
(b) B1 using a condom during sexual intercourse, oe
(b) B1 not sharing needles (e.g. needle exchange programmes for drug users), or screening donated blood/organs for HIV before use, oe
(b) Answer: Any two of: using condoms; not sharing needles; screening donated blood before use; taking antiretroviral drugs, which reduce the amount of virus in the blood.
(c) B1 HIV transmission generally requires close, direct contact or specific behaviours (e.g. unprotected sex, sharing needles) between people, oe
(c) B1 measles can be transmitted through the air by droplets, so it can spread between people who only have brief or indirect contact (e.g. being in the same room), oe
(c) B1 this means measles can generally spread more quickly through a population than HIV, which relies on more specific/less frequent routes of transmission, oe
(c) Answer: HIV needs close, direct contact or specific behaviours to spread, whereas measles can pass through the air between people with only brief contact. This means measles can generally spread through a population more quickly than HIV.

Question 6

(a) B1 bacterium cao
(a) Answer: Bacterium.
(b) B1 fever, oe
(b) B1 stomach cramps, vomiting or diarrhoea, oe
(b) Answer: Any two of: fever; stomach cramps; vomiting; diarrhoea.
(c) B1 vaccinating poultry reduces the number/proportion of birds that carry (are infected with) Salmonella bacteria, oe
(c) B1 this reduces the contamination of chicken meat and eggs with Salmonella bacteria during production, oe
(c) B1 so fewer bacteria are present in food reaching consumers, reducing the number of people who develop food poisoning after eating it, oe
(c) Answer: Vaccinating poultry reduces the number of birds carrying Salmonella, which reduces contamination of meat and eggs. This means fewer bacteria reach consumers in food, reducing cases of food poisoning.
(d) B1 cook the chicken thoroughly (until it is no longer pink and juices run clear), oe
(d) B1 avoid cross-contamination, e.g. use a separate chopping board for raw chicken and wash hands after handling it, or store chicken properly in the fridge, oe
(d) Answer: Any two of: cook the chicken thoroughly; avoid cross-contamination with other foods (e.g. use a separate chopping board and wash hands); store chicken correctly in the fridge.

Question 7

(a) B1 flame (sterilise) the inoculating loop before and after use, oe
(a) B1 only briefly lift the lid of the sterile agar plate, holding it at an angle, when inoculating it, to limit exposure to airborne microorganisms, oe (accept: work near a lit Bunsen burner; sterilise the agar/petri dish before use; tape the lid down with a few short pieces of tape rather than sealing it completely)
(a) Answer: Any two of: flame the inoculating loop before and after use; only briefly lift the lid at an angle when inoculating the plate; work close to a Bunsen burner flame; use a sterile (pre-sterilised) agar plate; secure the lid with a few short pieces of tape rather than sealing it completely.
(b) M1 2 hours = 120 minutes; number of divisions = 120 / 20 = 6, oe
(b) M1 method: 4 x 2⁶ (or repeated doubling shown: 4, 8, 16, 32, 64, 128, 256), oe
(b) A1 256 (bacteria) cao
(b) Answer: 256 bacteria
(c) B1 a real chopping board has far less nutrients available than nutrient agar, or bacteria compete for limited space/nutrients, or toxic waste products build up, so the bacteria could not keep dividing at the same rate, oe
(c) Answer: A real chopping board has much less nutrients available than nutrient agar (and bacteria would compete for limited space and nutrients), so the population would not keep doubling at this rate.
(d) B1 37 degrees C is closer to human body temperature, so incubating at this temperature is more likely to encourage the growth of pathogens that could infect humans, oe
(d) B1 incubating at a lower temperature (25 degrees C) makes the practical safer to carry out in a school laboratory, oe
(d) Answer: 37 degrees C is close to human body temperature, so it favours the growth of pathogens that could infect people. Incubating at 25 degrees C instead makes the practical safer in a school laboratory.
(e) B1 fewer colonies (less bacterial growth) would be seen on the plate incubated at 4 degrees C, oe
(e) B1 because at a lower temperature, bacterial reproduction (binary fission)/enzyme-controlled reactions happen more slowly, so the bacteria divide less often in the same time, oe
(e) Answer: Fewer colonies would grow at 4 degrees C, because bacterial reproduction (binary fission) happens more slowly at lower temperatures, so the bacteria divide less often in the same time.

Question 8

(a) B1 bacterium cao
(a) Answer: Bacterium.
(b) B1 by sexual contact (unprotected sex), oe
(b) Answer: By sexual contact (unprotected sex) with an infected person.
(c) B1 a thick yellow or green discharge from the vagina or penis, oe
(c) B1 pain (a burning sensation) when urinating, oe (accept: some infected people show no symptoms)
(c) Answer: A thick yellow or green discharge from the vagina or penis, and pain or a burning sensation when urinating.
(d) B1 random mutations in the bacteria's genes mean some individual bacteria are naturally more resistant to penicillin, oe
(d) B1 when penicillin is used, resistant bacteria survive and reproduce (passing on the resistance) while non-resistant bacteria are killed, so resistant bacteria become more common in the population over time (especially where antibiotics are overused or courses not completed), oe
(d) Answer: Random mutations mean some bacteria are naturally more resistant to penicillin. When penicillin is used, these resistant bacteria survive and reproduce while non-resistant bacteria are killed, so the proportion of resistant bacteria increases over time.

Question 9

(a) B1 a protist cao
(a) Answer: A protist (Plasmodium).
(b) B1 the (female) Anopheles mosquito cao
(b) Answer: The female Anopheles mosquito.
(c) B1 a mosquito bites (feeds on the blood of) a person infected with the malaria parasite, taking the parasite into its body along with the blood, oe
(c) B1 the parasite develops/multiplies inside the mosquito, oe
(c) B1 when the mosquito later bites an uninfected person, it injects saliva containing the parasite into that person's bloodstream, infecting them, oe
(c) Answer: A mosquito feeds on the blood of an infected person, taking in the parasite. The parasite develops inside the mosquito, and when the mosquito later bites another person, it injects saliva containing the parasite into their blood, infecting them.
(d) M1 decrease = 24500 - 6125 = 18375, oe
(d) M1 percentage decrease = (18375 / 24500) x 100, oe
(d) A1 75% cao
(d) Answer: 75% decrease
(e) B1 spraying insecticide (e.g. indoors, on walls) to kill mosquitoes, oe
(e) B1 draining or covering standing water where mosquitoes breed, or fitting window/door screens, or taking antimalarial drugs, oe
(e) Answer: Any two of: spraying insecticide to kill mosquitoes; draining or covering standing water where mosquitoes breed; fitting window and door screens; taking antimalarial prophylactic drugs.

Question 10

(a) B1 fungus cao
(a) Answer: Fungus.
(b) B1 purple or black spots/patches appear on the leaves, oe
(b) B1 the leaves surrounding the spots often turn yellow and drop off (fall) early, oe
(b) Answer: Purple or black spots appear on the leaves, and the leaves often turn yellow and drop off early.
(c) B1 losing leaves reduces the surface area/amount of chlorophyll available for photosynthesis, oe
(c) B1 this reduces the amount of glucose produced, so less energy/fewer new materials are available for growth, so the plant grows less (is stunted), oe
(c) Answer: Losing leaves reduces the surface area available for photosynthesis, so less glucose is made. This means less energy and fewer new materials are available for growth, so the plant grows less.
(d) B1 spray/treat the plant with a fungicide, oe
(d) B1 remove and destroy (e.g. burn) infected leaves promptly, rather than leaving them to fall and decay near the plant, oe
(d) Answer: Any two of: spray with a fungicide; remove and destroy infected leaves promptly; avoid overhead watering that splashes spores between plants.

Question 11

(a) B1 stunted growth, oe
(a) B1 spots on the leaves, patches of decay, malformed stems or leaves, growths, discolouration (e.g. yellowing), or the presence of pests, oe
(a) Answer: Any two of: stunted growth; spots on leaves; patches of decay; malformed stems or leaves; growths; discolouration (e.g. yellowing); presence of pests.
(b) B1 use a gardening manual or reference guide to compare the symptoms with pictures/descriptions of known plant diseases, oe
(b) B1 use an online database/website of common plant diseases, or use a testing kit (e.g. one that uses monoclonal antibodies) to identify a specific pathogen from a sample, oe
(b) Answer: Any two of: use a gardening manual or reference guide with pictures of infected plants; use an online database of common plant diseases; use a testing kit that identifies a pathogen using monoclonal antibodies.
(c) B1 if the affected plants are randomly scattered or found in patches that grow larger over time, this suggests a communicable pathogen, since disease spreads outward from an initial point of infection or is carried between plants (e.g. by wind, water or insects), oe
(c) B1 if the affected plants are found evenly across a distinct area of the field (e.g. matching a particular soil type or drainage pattern), this suggests a non-communicable cause such as a mineral ion deficiency, since this is not spread from plant to plant, oe
(c) B1 comparing the pattern seen in the field to these two possibilities allows the farmer to narrow down the likely cause before testing further, oe
(c) Answer: A random or spreading, patchy distribution suggests a communicable pathogen, since disease spreads outward from infected plants or is carried between them. An even distribution linked to a particular area of soil suggests a non-communicable cause, such as a mineral ion deficiency, since this does not spread plant to plant.

Question 12

(a) B1 physical: a waxy cuticle on the leaf surface, or a cellulose cell wall, or layers of dead cells (bark) around a stem, oe
(a) B1 mechanical: thorns or spines, or hairs, or leaves that droop or curl when touched, or mimicry, oe
(a) Answer: Physical, e.g. a waxy cuticle on the leaf (or cellulose cell walls). Mechanical, e.g. thorns and spines (or leaves that droop/curl when touched).
(b) B1 the waxy cuticle covers the leaf surface, making it difficult for pathogens to penetrate/enter the plant tissue, oe
(b) B1 it also reduces the amount of moisture left on the leaf surface, creating drier conditions that are less favourable for fungal spores to germinate, oe
(b) Answer: The waxy cuticle covers the leaf, forming a barrier that pathogens find difficult to penetrate. It also reduces moisture on the leaf surface, making conditions less favourable for fungal spores to germinate.
(c) B1 the poisonous chemicals deter or kill insects that would otherwise feed on the plant, oe
(c) B1 fewer insects feeding on the plant means fewer wounds/points of entry in its surface, and fewer opportunities for insects acting as vectors to introduce pathogens, reducing the risk of infection, oe
(c) Answer: The poisonous chemicals deter or kill insects that would feed on the plant. With fewer insects feeding, there are fewer wounds for pathogens to enter through and fewer chances for insect vectors to introduce pathogens.

Question 13

(a) M1 (35 / 250) x 100, oe
(a) A1 14% cao
(a) Answer: 14%
(b) M1 12000 x 0.14 (using their percentage from (a), ft), oe
(b) A1 1680 (plants) cao
(b) Answer: 1680 plants
(c) B1 advantage: no cost of buying/applying fungicide to the whole field, and no fungicide residues on the remaining crop, oe
(c) B1 disadvantage: removing plants reduces the total number of healthy plants left to harvest (yield), or the disease may already have spread to plants that look healthy so is not fully removed, oe
(c) Answer: Advantage: no cost of fungicide for the whole field and no fungicide residue on the crop. Disadvantage: removing plants reduces the number of healthy plants left to harvest, and some already-infected plants that look healthy may be missed.

Question 14

Level 3 (5-6): A detailed, logically structured comparison of the transmission and prevention of malaria and measles, leading to a clear, scientifically justified evaluation of which disease would be more difficult to control with limited healthcare funding. Correct scientific terminology is used throughout.
Level 2 (3-4): A description of how both malaria and measles are transmitted and prevented, with a simple comparison made between them and some attempt to judge which would be harder to control, though the reasoning may be incomplete or one-sided.
Level 1 (1-2): Basic, isolated statements are made about how malaria and/or measles are spread or prevented, with little or no comparison between the two diseases and no justified evaluation.
Level 0 (0): No relevant content.
Indicative content:
- Malaria is spread by a vector (the female Anopheles mosquito), which carries the Plasmodium protist between people when it feeds on blood.
- Measles is spread directly between people by droplet infection, through the air, from the coughs and sneezes of an infected person.
- Malaria prevention relies mainly on vector control: insecticide-treated bed nets, spraying insecticide, draining or covering standing water where mosquitoes breed, and antimalarial drugs.
- Measles prevention relies mainly on vaccination (the MMR vaccine) of the population, alongside isolating infected individuals to limit droplet spread.
- Vector control methods can be expensive to maintain, since nets need replacing, insecticide needs reapplying, and standing water can reappear, and mosquitoes can evolve resistance to insecticides over time.
- A vaccination programme can give long-lasting, population-wide protection from a single course of doses, which may become cheaper per person protected once the programme is established.
- However, vaccination programmes also have ongoing costs and challenges, such as cold-chain storage of vaccines and reaching people in remote areas.
- Malaria transmission depends on the local mosquito population, so control may need to continue indefinitely wherever suitable breeding sites and mosquitoes remain.
- A well-run measles vaccination programme can eventually reduce cases to a very low level (herd immunity), reducing the need for ongoing case-by-case control measures.
- A justified conclusion should be reached: for example, that malaria may be more difficult to control with limited funding because vector control needs continuous, repeated investment, whereas an established vaccination programme protects the population with lower ongoing cost per case prevented; or the reverse view, provided it is supported by scientifically accurate comparison and reasoning.
- Credit any scientifically accurate evaluation that is clearly supported by a comparison of the transmission and prevention methods of both diseases.

Question 15

(a) M1 number of doublings = 12 / 3 = 4, oe
(a) M1 method: 8 x 2⁴ (or sequential doubling: 8, 16, 32, 64, 128), oe
(a) A1 128 (infected plants) cao
(a) Answer: 128 infected plants
(b) M1 new infections from day 12 to day 22 = 5 x 10 = 50 (using their (a) as the day-12 total, ft), oe
(b) A1 178 (128 + 50) cao (ft from (a))
(b) Answer: 178 plants had become infected in total by day 22.
(c) B1 use drip irrigation (watering the soil/roots directly) instead of overhead sprinklers, to avoid water splashing between plants, oe
(c) B1 increase the spacing between plants, to reduce plant-to-plant contact and the distance water can splash between them, oe
(c) Answer: Any two of: use drip irrigation instead of overhead sprinklers, to avoid water splash between plants; increase the spacing between plants, to reduce contact and splash distance; water earlier in the day so leaves dry faster.

Question 16

(a) B1 400 (x400) cao
(a) Answer: x400 (400 times magnification).
(b) M1 converts image size to um: 15 mm x 1000 = 15000 um, oe
(b) M1 actual size = image size / magnification = 15000 / 300, oe
(b) A1 50 um cao
(b) Answer: 50 micrometres (50 um)
(c) B1 visible symptoms alone can look similar to those of other diseases, pests or nutrient deficiencies, oe
(c) B1 so examining the pathogen itself (e.g. seeing fungal hyphae or spores) under the microscope confirms the specific cause, allowing the correct treatment to be chosen, oe
(c) Answer: Visible symptoms can look similar for different diseases, pests or deficiencies. Confirming the pathogen under the microscope identifies the specific cause, so the correct treatment can be used.
(d) B1 lower the cover slip gently onto the sample at an angle (using a mounted needle), to avoid trapping air bubbles and crushing the sample, oe
(d) Answer: Lower the cover slip gently onto the sample at an angle, using a mounted needle, to avoid trapping air bubbles or crushing the sample.

Communicable Diseases and Pathogens - Worksheets, Questions and Revision