📚 4.1 Communicable Diseases and Disease Prevention: Experimental Design | 4.1 传染病与疾病预防：实验设计
Experimental design lies at the core of understanding how communicable diseases spread and how we can evaluate strategies to prevent them. From testing disinfectants in the laboratory to modelling the transmission of a virus in a classroom simulation, well-planned experiments provide reliable evidence that informs public health decisions. This article explores the key principles of designing investigations related to infectious diseases, highlighting practical lab work, data analysis, and ethical considerations that are central to the A-level Biology specification.
实验设计是理解传染病如何传播以及如何评估预防策略的核心。从在实验室中测试消毒剂到在课堂模拟中建立病毒传播模型，精心设计的实验能够提供可靠证据，为公共卫生决策提供依据。本文探讨设计传染病相关研究的关键原则，重点介绍与A-level生物大纲密切相关的实践操作、数据分析及伦理考量。
1. The Role of Experiments in Understanding Infectious Diseases | 实验在理解传染病中的作用
Communicable diseases are caused by pathogens such as bacteria, viruses, fungi and protists. Experiments allow us to investigate how these microorganisms survive, reproduce and spread between hosts. By systematically manipulating variables, we can test hypotheses about the effectiveness of hand-washing, the potency of antibiotics, and the impact of vaccination programmes. Without controlled experiments, our understanding would rely solely on observational studies, which often cannot establish causation.
传染病由细菌、病毒、真菌和原生生物等病原体引起。实验使我们能够研究这些微生物如何存活、繁殖以及在宿主之间传播。通过系统地操纵变量，我们可以检验关于洗手有效性、抗生素效力和疫苗接种计划影响的假设。没有对照实验，我们的理解将仅依赖于观察性研究，而后者通常无法确定因果关系。
In the context of disease prevention, experimental design helps answer questions such as: “How quickly does a disease spread in a population with different levels of immunity?” or “Which concentration of a disinfectant is sufficient to kill a specific pathogen?” These questions are directly linked to the learning outcomes for communicable diseases, where students are expected to design and evaluate investigations.
在疾病预防的背景下，实验设计有助于回答诸如”在具有不同免疫水平的人群中，疾病传播速度有多快？”或”哪种浓度的消毒剂足以杀死特定病原体？”等问题。这些问题与传染病的学习目标直接相关，而学生需要能够设计并评估相关研究。
2. Core Principles of Experimental Design | 实验设计的核心原则
Any reliable investigation into communicable diseases must adhere to the basic principles of experimental design. First, a clear independent variable must be identified, such as the type of disinfectant, the temperature of incubation, or the level of herd immunity in a simulation. The dependent variable should be measurable, for example the diameter of the zone of inhibition, the number of bacterial colonies, or the proportion of “infected” individuals in a model.
任何可靠的传染病研究都必须遵循实验设计的基本原则。首先，必须明确自变量，例如消毒剂的类型、培养温度或在模拟中的群体免疫水平。因变量应当可测量，例如抑菌圈的直径、细菌菌落的数量或模型中”感染”个体的比例。
Control groups are essential: a disc soaked in sterile water alongside disinfectant discs, or a group of unvaccinated participants in a vaccine trial. All other variables must be controlled to ensure a fair test. In microbiology, this includes using the same bacterial species, standardising inoculum size, maintaining consistent incubation time and temperature, and using aseptic techniques to prevent contamination. Replicates should be performed to assess the reliability of results and to allow calculation of means and standard deviations.
对照组必不可少：与消毒剂纸片一起放置的浸泡无菌水的纸片，或在疫苗试验中未接种疫苗的参与者组。所有其他变量必须加以控制，以确保公平测试。在微生物学中，这包括使用相同的细菌菌种、标准化接种量、保持一致的培养时间和温度，以及使用无菌操作技术防止污染。应进行重复实验以评估结果的可靠性，并允许计算平均值和标准差。
3. Aseptic Technique: The Foundation of Microbiological Experiments | 无菌技术：微生物实验的基础
Before designing any experiment that involves culturing microorganisms, it is vital to master aseptic techniques. These practices prevent unwanted contamination of cultures and, equally importantly, protect the experimenter from potentially harmful pathogens. Key steps include disinfecting the work surface with a suitable disinfectant before and after work, using a Bunsen burner to create an updraft that reduces airborne contamination, and flaming the neck of culture bottles and the inoculating loop.
在设计任何涉及培养微生物的实验之前，掌握无菌技术至关重要。这些操作能防止培养物被意外污染，同样重要的是能保护实验者免受潜在有害病原体的侵害。关键步骤包括：在工作前后用合适的消毒剂对工作台进行消毒，使用本生灯形成上升气流以减少空气传播的污染，以及对培养瓶管口和接种环进行灼烧灭菌。
When designing an experiment, you must incorporate aseptic technique as a standard procedure. For instance, when inoculating agar plates, lift the lid only slightly and for the minimum time necessary. All equipment that comes into contact with microbes must be sterilised before use. Disposal of used cultures must follow strict safety protocols, usually by autoclaving. These controls ensure that any observed effects are due to the independent variable rather than contamination.
在设计实验时，必须将无菌技术作为标准程序纳入。例如，在接种琼脂平板时，只应稍微抬起皿盖并尽量缩短暴露时间。所有与微生物接触的设备在使用前都必须灭菌。用过的培养物必须按照严格的安全规程处理，通常通过高压蒸汽灭菌。这些控制措施确保任何观察到的效应均来自自变量，而非污染。
4. Investigating the Effect of Disinfectants and Antiseptics | 探究消毒剂和抗菌剂的效果
A classic experiment is to compare the effectiveness of different disinfectants or antiseptics using the disc diffusion method. Agar plates are uniformly inoculated with a standardised volume of a bacterial culture, such as Escherichia coli or Staphylococcus epidermidis. Filter paper discs are saturated with equal volumes of the test substances, including a distilled water control, and placed evenly on the agar surface. After incubation at a suitable temperature (e.g. 30 °C or 37 °C) for 24–48 hours, the diameters of any clear zones around the discs are measured.
经典的实验是使用纸片扩散法比较不同消毒剂或抗菌剂的效果。将标准体积的细菌培养液（例如大肠杆菌或表皮葡萄球菌）均匀涂抹在琼脂平板上。用等体积的供试品溶液浸透滤纸片，包括一个蒸馏水对照，并均匀地放置在琼脂表面。在适宜温度（如30°C或37°C）下培养24–48小时后，测量纸片周围任何透明抑菌圈的直径。
In designing this experiment, careful attention must be paid to controlling variables. The age and species of the bacterial culture must be kept constant, the volume and concentration of the disinfectant must be precise, and the discs should be of identical size and absorbency. The experiment should be repeated at least three times to ensure that the results are reproducible. Students are often asked to evaluate the limitations of this technique, such as differences in diffusion rates of the substances in agar and the fact that the zone of inhibition does not necessarily equal the killing ability of the agent.
在设计该实验时，必须仔细控制变量。细菌培养物的菌龄和种类必须保持一致，消毒剂的体积和浓度需精确，纸片应具有相同的大小和吸水性。实验至少应重复三次，以确保结果的再现性。学生常常被要求评估这一技术的局限性，例如不同物质在琼脂中的扩散速率差异，以及抑菌圈的大小并不完全等同于药剂的杀菌能力。
You could extend the design by investigating the effect of varying the concentration of a single disinfectant. Prepare a dilution series of the disinfectant and measure the diameter of the inhibition zone for each concentration. Plot a calibration curve to find the minimum inhibitory concentration (MIC). This demonstrates how experimental design can be refined to yield quantitative data that informs safe usage levels for hospital or household disinfection.
你可以通过探究单一消毒剂不同浓度的影响来拓展实验设计。制备消毒剂的稀释系列，并测量每个浓度的抑菌圈直径。绘制标准曲线以得出最低抑菌浓度（MIC）。这表明实验设计可以如何优化，以产生定量数据，从而为医院或家庭消毒的安全使用水平提供信息。
5. Designing a Hand-Washing Effectiveness Experiment | 洗手有效性实验设计
Evaluating the impact of hand-washing is a practical way to connect experimental design to everyday disease prevention. A simple design involves pressing unwashed fingers onto a nutrient agar plate, then repeating the process after washing hands with a particular soap or sanitizer. After incubation, the number of bacterial and fungal colonies can be compared. A control group might use fingers that were only rinsed with sterile water.
评估洗手的影响是将实验设计与日常疾病预防联系起来的一种实用方法。一种简单的设计是先将未洗过的手指按压在营养琼脂平板上，然后用特定的肥皂或洗手液洗手后重复操作。培养后，可以比较细菌和真菌菌落的数量。可设置一组仅用无菌水冲洗手指的对照组。
When designing this investigation, it is critical to standardise the pressure and duration of finger contact with the agar. Splitting the plate into sections and using the same fingers from a single volunteer helps reduce variability. Variables to control include the type and volume of soap, the duration and technique of hand-washing, and the temperature of the water. Multiple volunteers can be used, but this introduces biological variation that must be accounted for in analysis, perhaps by pairing each person’s unwashed and washed samples.
在设计此研究时，标准化的手指接触琼脂的压力和持续时间至关重要。将平板分成区域并使用同一志愿者的相同手指有助于减少变异。需要控制的变量包括肥皂的类型和用量、洗手的时长和手法以及水温。可使用多名志愿者，但这会引入生物学差异，分析时需加以考虑，例如将每个人的未洗手与洗手样本进行配对。
The results not only illustrate the immediate reduction in microbial load but also open up discussions about why hand-washing breaks the chain of infection for enteric and respiratory pathogens. Students can propose further experiments, such as testing different types of hand-drying methods or comparing alcohol-based sanitizers to soap and water.
实验结果不仅说明了微生物负荷的直接减少，还引发了对洗手为何能打断肠道和呼吸道病原体感染链的讨论。学生可以提出进一步的实验，例如测试不同的干手方法或比较含酒精洗手液与肥皂水。
6. Modelling Disease Transmission in the Laboratory | 在实验室中模拟疾病传播
Because it is unethical to deliberately infect people with pathogens, modelling the spread of a communicable disease using a harmless chemical or digital simulation is a powerful experimental design. A common classroom model involves each student being given a small vial of liquid—most contain water, but one or a few contain a dilute sodium hydroxide (NaOH) solution, representing the “infectious” agent. Students exchange liquids by transferring drops with a pipette, simulating random contacts. After several rounds, a few drops of phenolphthalein indicator are added to all vials; a pink colour reveals those who have become “infected”.
由于故意让人感染病原体是不道德的，因此使用无害的化学物质或数字模拟来模拟传染病的传播是一种强有力的实验设计。一个常见的课堂模型是给每个学生一小管液体——大部分是水，但其中一支或几支含有稀氢氧化钠（NaOH）溶液，代表”传染性”病原体。学生们通过用移液管转移液滴来交换液体，模拟随机接触。几轮之后，向所有试管中加入几滴酚酞指示剂；变成粉红色的液体表示那些已经”感染”的人。
This experimental design allows the manipulation of several independent variables: the number of initially infected individuals, the number of exchanges per round, or the introduction of a “preventive” measure such as some individuals using a barrier (e.g. a small filter) or not participating in an exchange (isolation). The dependent variable is the final proportion of the population that becomes infected. Data can be collected across multiple runs to calculate mean attack rates and to illustrate concepts like herd immunity threshold.
这一实验设计允许操控多个自变量：初始感染者的数量、每轮交换的次数，或引入”预防”措施，例如某些个体使用屏障（如小过滤器）或不参与交换（隔离）。因变量是最终被感染的人口比例。可以在多次运行中收集数据，计算平均侵袭率，并说明群体免疫阈值等概念。
When planning such a model, it is important to define the transmission parameters clearly, use sterile or clean equipment, and ensure that the indicator is sensitive enough. The model can be compared with real epidemiological data to discuss its strengths and limitations, such as the assumption of random mixing versus real-world network structures. This type of experimental design bridges theoretical biology and practical investigation.
在设计此类模型时，重要的是清晰地定义传播参数，使用无菌或清洁的设备，并确保指示剂的灵敏度足够。可以将该模型与真实的流行病学数据进行比较，讨论其优势和局限性，例如随机混合的假设与现实世界网络结构之间的差异。这种实验设计搭建了理论生物学与实践探究之间的桥梁。
7. Testing Antibiotic Sensitivity Using the Disc Diffusion Method | 使用纸片扩散法测试抗生素敏感性
Antibiotic sensitivity testing is fundamental in clinical settings and a standard experiment in school laboratories. Bacteria such as Staphylococcus albus or Micrococcus luteus are spread on Mueller-Hinton agar plates. Antibiotic-impregnated discs, each containing a known amount of a different antibiotic, are placed on the agar. After incubation, the zones of inhibition are measured and compared with standard interpretation charts to classify the bacteria as sensitive, intermediate, or resistant to each antibiotic.
抗生素敏感性测试在临床环境中至关重要，也是学校实验室的标准实验。将如表皮葡萄球菌或藤黄微球菌等细菌涂布在Mueller-Hinton琼脂平板上。将浸渍有抗生素的纸片（每片含有已知量的不同抗生素）放置在琼脂上。培养后，测量抑菌圈并与标准判读图表进行比较，以将细菌分为对该抗生素敏感、中介或耐药。
In designing this test, uniform inoculum density is critical and is often standardised to a 0.5 McFarland turbidity standard. The depth of the agar, the moisture content, and the incubation atmosphere (aerobic or with 5% CO₂) must be controlled. A negative control disc containing no antibiotic but the same solvent should be used to confirm that any inhibition is due to the antibiotic itself. The experiment can be extended by testing the same antibiotic at different concentrations to produce a dose-response curve, reinforcing the concept of minimum bactericidal concentration (MBC).
在设计此测试时，一致的接种密度至关重要，通常标准化至0.5 McFarland浊度标准。琼脂的厚度、含水量和培养环境（有氧或含5% CO₂）必须加以控制。应使用不含抗生素但含相同溶剂的阴性对照纸片，以确认任何抑制作用都来自抗生素本身。该实验可通过测试不同浓度的同一种抗生素来扩展，生成剂量-反应曲线，从而强化最小杀菌浓度（MBC）的概念。
This experiment has direct relevance to the problem of antibiotic resistance. Students can design comparative studies using wild-type and resistant strains, or test the effect of combining antibiotics. Such investigations encourage critical thinking about how experimental design must adapt when dealing with genetically diverse pathogens.
该实验与抗生素耐药性问题直接相关。学生可以设计使用野生型和耐药菌株的比较研究，或测试联合使用抗生素的效果。此类研究鼓励批判性思考，即在处理遗传多样的病原体时，实验设计必须如何调整。
8. Factors Affecting Microbial Growth: A Controlled Experiment | 影响微生物生长的因素：对照实验
To prevent communicable diseases, it is useful to understand the environmental factors that influence pathogen growth. A controlled experiment can be designed to investigate the effect of temperature, pH, oxygen availability, or nutrient concentration on the growth of a microorganism. For instance, inoculate identical broth cultures of E. coli and incubate them at a range of temperatures (10 °C, 20 °C, 30 °C, 40 °C, 50 °C). Use a colorimeter or viable cell counts to measure growth at regular intervals.
为了预防传染病，了解影响病原体生长的环境因素是有益的。可以设计对照实验，探究温度、pH值、氧气供应或营养浓度对微生物生长的影响。例如，接种相同的大肠杆菌肉汤培养物，并在不同温度（10°C、20°C、30°C、40°C、50°C）下培养。使用比色计或活菌计数定期测量生长情况。
The independent variable (temperature) must be precisely controlled using water baths or incubators, and all other variables—such as the volume of broth, the initial cell concentration, and the shaking speed—must be kept constant. Aseptic transfers are necessary when taking samples to avoid introducing contaminants. The results often produce a classic bell-shaped curve, allowing students to identify the optimum temperature and the minimum and maximum growth temperatures. This type of experiment helps explain why refrigeration and proper cooking temperatures are critical in food safety and disease prevention.
自变量（温度）必须使用水浴或培养箱精确控制，所有其他变量——如肉汤的体积、初始细胞浓度和振荡速度——必须保持一致。取样时需进行无菌操作，以免带入污染物。结果通常会形成经典的钟形曲线，学生由此可确定最适温度以及最低和最高生长温度。这类实验有助于解释冷藏和适当烹调温度在食品安全和疾病预防中为何至关重要。
9. Simulating Vaccination and Herd Immunity | 模拟疫苗接种与群体免疫
Simulations are powerful tools for investigating the impact of vaccination programmes on the spread of a disease. Building on the disease transmission model with NaOH and phenolphthalein, you can designate a certain proportion of the class as “vaccinated” from the start. These individuals receive a vial that contains a neutralising substance (e.g. a weak acid) that will not produce a pink colour even if they receive the “infectious” NaOH during exchanges. By altering the percentage of vaccinated individuals and running multiple rounds, you can observe the threshold at which an outbreak fails to establish.
模拟是研究疫苗接种计划对疾病传播影响的强大工具。在基于氢氧化钠和酚酞的疾病传播模型基础上，一开始即可指定班级中的一定比例为”已接种疫苗”者。这些个人拿到的试管中含有中和物质（例如弱酸），即使他们在交换过程中接收到了”感染性的”NaOH也不会显示粉红色。通过改变接种比例并进行多轮操作，可以观察到疫情无法建立的阈值。
From an experimental design perspective, just like in real epidemiological trials, it is important to ensure that “vaccinated” and “unvaccinated” individuals are randomly assigned and that they interact in the same way. Data collection involves recording the final number of “infections” for each level of vaccination coverage. The results can be plotted to show the relationship between vaccination rate and outbreak size, clearly demonstrating the concept of herd immunity—when a high enough proportion of the population is immune, the pathogen cannot spread effectively.
从实验设计的角度来看，与真实的流行病学试验一样，重要的是确保”接种”和”未接种”个体是随机分配的，并且他们的互动方式相同。数据收集包括记录每个疫苗接种覆盖率水平下的最终”感染”人数。结果可以绘制成图，显示接种率与疫情规模之间的关系，清晰地展示群体免疫的概念——当人口中有足够高的比例具有免疫力时，病原体无法有效传播。
Students can extend this design by modelling waning immunity over time or by simulating the impact of a booster vaccination programme. Such extensions require careful planning of additional variables and clear success criteria, reinforcing higher-order experimental design skills.
学生可以通过模拟免疫力随时间减弱或模拟加强针接种规划的影响来扩展这一设计。此类扩展需要仔细规划额外的变量和明确的成功标准，从而强化高阶实验设计技能。
10. Epidemiological Studies and Experimental Design | 流行病学调查与实验设计
Not all investigations into disease prevention take place in a laboratory; field-based epidemiological studies rely heavily on sound experimental or quasi-experimental designs. A cohort study follows a group of individuals over time to see who develops a disease, comparing those exposed to a risk factor with those not exposed. For example, comparing the incidence of tuberculosis in people who live in overcrowded housing versus those who do not. This is an observational design, but careful control of confounding variables such as age and nutrition is essential.
并非所有疾病预防研究都在实验室中进行；基于现场的流行病学研究在很大程度上依赖于良好的实验或准实验设计。队列研究是在一段时间内跟踪一组个体，观察谁患病，并比较暴露于危险因素和未暴露的个体。例如，比较居住在拥挤住房中者与未居住在其中者的结核病发病率。这是一种观察性设计，但仔细控制年龄和营养等混杂变量至关重要。
Case-control studies, which compare patients who have a disease with controls who do not, are often used for rare diseases or outbreaks. The design must carefully match cases and controls for confounding variables. In the context of the A-level specification, students are expected to recognise these study designs and understand how they can be used to identify risk factors for communicable diseases, though the practical emphasis remains on experimental manipulations in the lab.
病例对照研究将患有疾病者与未患病的对照组进行比较，常用于罕见疾病或暴发调查。设计必须仔细匹配病例和对照的混杂变量。在A-level大纲中，学生需要认识这些研究设计，并理解它们如何用于识别传染病的危险因素，但实践重点仍放在实验室中的实验操作上。
11. Data Analysis, Statistics, and Drawing Conclusions | 数据分析、统计与得出结论
A well-designed experiment yields data that must be analysed appropriately. For microbiological experiments, calculate the mean zone of inhibition from replicates and use standard deviation or range to express variability. A bar chart can visualise the effectiveness of different disinfectants. In the case of disease transmission models, you might calculate the percentage of infections in each round and test whether the difference between vaccinated and unvaccinated groups is statistically significant using a chi-squared test or a t-test, depending on the nature of the data.
精心设计的实验会产生必须进行恰当分析的数据。对于微生物实验，根据重复实验计算平均抑菌圈直径，并使用标准差或极差来表示变异。柱状图可以直观显示不同消毒剂的效果。对于疾病传播模型，你可以计算每轮感染的百分比，并根据数据性质使用卡方检验或t检验来检验接种组和未接种组之间的差异是否具有统计学显著性。
When writing the conclusion, always refer back to the original hypothesis. Discuss whether the data support or refute it, and consider the reliability of the results. Identify any anomalous data points and suggest explanations. Most importantly, evaluate the experimental design itself—were all variables truly controlled? Could the precision of measurements be improved? What sources of systematic or random error might have affected the outcome? This critical evaluation is a key skill in A-level practical assessment.
在撰写结论时，务必回溯最初的假设。讨论数据是支持还是反驳了该假设，并考虑结果的可靠性。识别任何异常数据点并提出解释。最重要的是，评估实验设计本身——是否真正控制了所有变量？测量的精密度是否可以改进？哪些系统误差或随机误差的来源可能影响了结果？这种批判性评价是A-level实践评估中的一项关键技能。
12. Limitations and Ethical Considerations | 局限性及伦理考量
All experimental designs have limitations, and it is important to acknowledge them explicitly. In the disc diffusion assay, the size of the zone of inhibition is influenced not only by the antimicrobial activity but also by the molecule’s size and solubility, which affect diffusion through the agar. In the hand-washing experiment, individual variation in skin flora and the difficulty of controlling pressure when pressing fingers onto agar introduce variability. In the disease transmission simulation, the simplified mixing pattern may not reflect real-world social networks, and the indicator reaction is not a true biological infection.
所有实验设计都存在局限性，明确承认这些局限非常重要。在纸片扩散法中，抑菌圈的大小不仅受抗菌活性影响，还受分子大小和溶解度的影响，后者影响在琼脂中的扩散。在洗手实验中，皮肤菌群的个体差异以及手指按压琼脂时难以控制压力会引入变异性。在疾病传播模拟中，简化的混合模式可能不能反映真实世界的社会网络，且指示剂反应并非真正的生物学感染。
Ethical considerations must also be addressed when designing experiments involving living organisms. In the UK, schools follow guidelines such as CLEAPSS, which specify that pathogens known to cause serious disease must not be cultured, incubation should generally be below 30 °C to avoid selecting for human pathogens, and all microbial waste must be autoclaved before disposal. When using animal models or human volunteers, informed consent and minimal risk must be prioritised. A strong experimental design always balances the pursuit of knowledge with the responsibility to do no harm.
在设计涉及活体生物的实验时，还必须考虑伦理因素。在英国，学校遵循诸如CLEAPSS之类的指南，其中规定不得培养已知会导致严重疾病的病原体，培养温度通常应低于30°C以避免筛选出人类病原体，且所有微生物废弃物在丢弃前必须进行高压蒸汽灭菌。在使用动物模型或人类志愿者时，必须优先考虑知情同意和最小风险。一个严谨的实验设计始终要在追求知识与不造成伤害的责任之间取得平衡。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导