Communicable Diseases & Disease Prevention | 传染病与疾病预防

📚 Communicable Diseases & Disease Prevention | 传染病与疾病预防

Communicable diseases, also known as infectious diseases, are illnesses caused by pathogens that can be transmitted directly or indirectly from one host to another. Understanding the nature of these diseases, their causative agents, and the strategies for prevention is a cornerstone of AS Biology and essential for public health worldwide.

传染病，又称感染性疾病，是由病原体引起的、可直接或间接在宿主之间传播的疾病。了解这些疾病的性质、致病因子及预防策略，是AS生物学的核心内容，也是全球公共卫生的基础。

1. What Are Communicable Diseases | 什么是传染病

A communicable disease is any disease caused by a biological agent (pathogen) that can be transmitted from an infected individual to a susceptible host. Transmission may occur through direct contact, airborne droplets, contaminated food or water, or via vectors such as insects.

传染病是由生物性因子（病原体）引起的、可从受感染个体传播至易感宿主的疾病。传播途径包括直接接触、空气飞沫、受污染的食物或水，以及昆虫等媒介。

Key characteristics of communicable diseases include their ability to spread within populations, the presence of an incubation period, and the potential to cause epidemics or pandemics. The study of communicable diseases focuses on the pathogen involved, the route of transmission, and the host’s immune response.

传染病的关键特征包括：能在人群中传播、具有潜伏期、以及可能引发流行病或大流行。传染病的研究主要关注致病病原体、传播途径和宿主的免疫反应。

2. Types of Pathogens | 病原体的类型

Pathogens are microorganisms that cause disease. There are four main categories of pathogens studied at AS level: bacteria, viruses, fungi, and protoctists (protozoa). Each type has distinct structural features, modes of reproduction, and mechanisms of causing disease.

病原体是引起疾病的微生物。AS阶段主要学习四类病原体：细菌、病毒、真菌和原生生物（原生动物）。每种类型都具有独特的结构特征、繁殖方式和致病机制。

Pathogen Type	Structure	Example Disease	Treated With
Bacteria	Prokaryotic cells with cell walls, no nucleus	Cholera, TB	Antibiotics
Viruses	Nucleic acid core with protein coat, no cellular structure	HIV/AIDS, Influenza	Antivirals (limited)
Fungi	Eukaryotic cells with chitin cell walls, hyphae	Athlete’s foot, ringworm	Antifungals
Protoctists	Eukaryotic single-celled organisms	Malaria	Antiprotozoals

Bacteria are prokaryotic organisms that lack a true nucleus. They reproduce by binary fission and can produce toxins that damage host tissues. Viruses are much smaller than bacteria and are obligate intracellular parasites—they can only replicate inside a host cell, using the host’s metabolic machinery.

细菌是缺乏真正细胞核的原核生物，通过二分裂方式繁殖，并可产生破坏宿主组织的毒素。病毒比细菌小得多，是专性胞内寄生生物——仅在宿主细胞内，利用宿主的代谢机制进行复制。

3. Bacterial Disease: Cholera | 细菌性疾病：霍乱

Cholera is an acute diarrhoeal disease caused by the bacterium Vibrio cholerae. It is transmitted through the faecal-oral route, typically via contaminated drinking water or food. The disease is most common in areas with poor sanitation and limited access to clean water.

霍乱是由霍乱弧菌（Vibrio cholerae）引起的急性腹泻性疾病。它通过粪-口途径传播，通常经由受污染饮用水或食物感染。该疾病在卫生条件差、清洁水供应有限的地区最为常见。

Once ingested, Vibrio cholerae colonises the small intestine and secretes an enterotoxin. This toxin causes the intestinal epithelial cells to over-secrete chloride ions into the gut lumen, leading to massive water loss through osmosis. The result is severe watery diarrhoea (‘rice-water’ stools) that can cause rapid dehydration and electrolyte imbalance, leading to death if untreated.

霍乱弧菌进入人体后，在小肠定殖并分泌肠毒素。该毒素导致肠上皮细胞向肠腔过度分泌氯离子，通过渗透作用引发大量水分流失。其结果是严重的米泔水样腹泻，可迅速导致脱水和电解质紊乱，若不治疗可能致命。

Treatment for cholera primarily involves oral rehydration therapy (ORT) to replace lost fluids and electrolytes. Antibiotics such as tetracycline may also be administered to shorten the duration of the illness, though the key priority is fluid replacement.

霍乱的治疗主要采用口服补液疗法（ORT），以补充流失的水分和电解质。也可使用四环素等抗生素缩短病程，但首要任务是补液。

4. Bacterial Disease: Tuberculosis | 细菌性疾病：结核病

Tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis. It primarily affects the lungs (pulmonary TB) but can spread to other organs. TB is transmitted through airborne droplets when an infected person coughs, sneezes, or talks.

结核病（TB）由结核分枝杆菌（Mycobacterium tuberculosis）引起，主要侵犯肺部（肺结核），但也可扩散至其他器官。结核病经空气飞沫传播，当感染者咳嗽、打喷嚏或说话时即可传染他人。

After inhalation, the bacteria are engulfed by macrophages in the alveoli. However, M. tuberculosis can survive inside macrophages and evade the immune response. Infected individuals may develop active disease with symptoms such as persistent cough, fever, night sweats, weight loss, and coughing up blood. Alternatively, the bacteria may remain dormant in a latent state for years.

结核分枝杆菌被吸入后，会被肺泡中的巨噬细胞吞噬。然而，它能在巨噬细胞内存活并逃避免疫应答。感染者可能发展为活动性疾病，出现持续咳嗽、发热、盗汗、体重减轻和咯血等症状；也可能使细菌长期处于潜伏状态、暂不发病。

TB is treated with a long course of multiple antibiotics (e.g. rifampicin and isoniazid) over 6-9 months. The emergence of multidrug-resistant TB (MDR-TB) is a major global health concern, highlighting the importance of completing the full treatment course and developing new therapeutic agents.

结核病需使用多种抗生素（如利福平和异烟肼）进行6-9个月的长期疗程。耐多药结核病（MDR-TB）的出现是重大全球健康挑战，突显了完成全程治疗和开发新药的重要性。

5. Viral Disease: HIV/AIDS | 病毒性疾病：艾滋病

Human Immunodeficiency Virus (HIV) is a retrovirus that causes Acquired Immunodeficiency Syndrome (AIDS). HIV is transmitted through direct contact with infected body fluids, including blood, semen, vaginal secretions, and breast milk. Common routes include unprotected sexual intercourse, sharing contaminated needles, and mother-to-child transmission during pregnancy, childbirth, or breastfeeding.

人类免疫缺陷病毒（HIV）是一种逆转录病毒，可导致获得性免疫缺陷综合征（AIDS，即艾滋病）。HIV通过接触感染者的体液传播，包括血液、精液、阴道分泌物和母乳。常见传播途径包括无保护性行为、共用受污染针具，以及妊娠、分娩或母乳喂养期间的母婴传播。

HIV specifically infects helper T cells (CD4⁺ T lymphocytes), which play a central role in coordinating the immune response. The virus enters the cell, reverse-transcribes its RNA genome into DNA, and integrates this viral DNA into the host cell genome. The infected cells eventually die, causing a progressive decline in helper T cell numbers.

HIV特异性感染辅助性T细胞（CD4⁺ T淋巴细胞），而此类细胞在协调免疫应答中起核心作用。病毒进入细胞后，将其RNA基因组逆转录为DNA，并将病毒DNA整合到宿主细胞基因组中。受感染的细胞最终死亡，导致辅助性T细胞数量进行性下降。

As the immune system becomes progressively impaired, the individual becomes susceptible to opportunistic infections such as pneumonia, tuberculosis, and certain cancers. AIDS is the final stage of HIV infection, characterised by a CD4⁺ T cell count below 200 cells per mm³ and the presence of opportunistic infections.

随着免疫系统逐渐受损，患者易感染机会性疾病，如肺炎、结核病和某些癌症。艾滋病是HIV感染的最终阶段，其特征是CD4⁺ T细胞计数低于200个/mm³，并伴有机会性感染。

There is currently no cure for HIV, but antiretroviral therapy (ART) can effectively suppress viral replication, allowing individuals to live longer, healthier lives and reducing the risk of transmission. Prevention strategies include condom use, needle exchange programmes, and pre-exposure prophylaxis (PrEP).

目前艾滋病尚无法治愈，但抗逆转录病毒治疗（ART）可有效抑制病毒复制，使患者能够更长寿、更健康地生活，并降低传播风险。预防策略包括使用安全套、针具交换项目和暴露前预防（PrEP）。

6. Protoctist Disease: Malaria | 原生生物疾病：疟疾

Malaria is caused by protoctist parasites of the genus Plasmodium, with Plasmodium falciparum being the most deadly species. Malaria is transmitted by the bite of female Anopheles mosquitoes, which act as the biological vector for the parasite.

疟疾由疟原虫属（Plasmodium）的原生生物寄生虫引起，其中恶性疟原虫（Plasmodium falciparum）是最致命的物种。疟疾通过雌性按蚊（Anopheles）叮咬传播，按蚊是疟原虫的生物媒介。

The life cycle of Plasmodium involves two hosts. In the mosquito, sexual reproduction occurs and sporozoites are produced. When the mosquito bites a human, sporozoites enter the bloodstream and travel to the liver, where they invade hepatocytes and multiply asexually. After a period of maturation, merozoites are released into the blood and infect red blood cells. The rupture of infected red blood cells releases toxins and causes cyclical fever, chills, and anaemia.

疟原虫的生命周期涉及两个宿主。在蚊体内进行有性生殖并产生子孢子；蚊子叮咬人时，子孢子进入血流并移至肝脏，侵入肝细胞进行无性增殖。经过一段成熟期后，裂殖子释放入血并感染红细胞。受感染红细胞的破裂释放毒素，导致周期性发热、寒战和贫血。

Malaria prevention focuses on controlling the mosquito vector: using insecticide-treated bed nets (ITNs), indoor residual spraying (IRS), eliminating standing water where mosquitoes breed, and taking antimalarial prophylaxis medications when travelling to endemic areas.

疟疾预防重点在于控制蚊媒：使用经杀虫剂处理的蚊帐（ITNs）、室内滞留喷洒（IRS）、清除蚊虫繁殖的积水，以及前往疫区旅行时服用抗疟预防药物。

7. Modes of Transmission | 传播方式

Understanding how communicable diseases spread is essential for designing effective prevention strategies. There are four main modes of transmission to consider at AS level.

了解传染病的传播方式是设计有效预防策略的基础。AS阶段需要掌握四种主要传播方式。

Direct contact transmission — physical contact with an infected person, such as touching, kissing, or sexual contact. Examples include HIV and skin infections.

直接接触传播——与感染者发生身体接触，如触摸、亲吻或性接触。例如HIV和皮肤感染。
Airborne/droplet transmission — inhalation of droplets expelled when an infected person coughs or sneezes. Examples include TB and influenza.

空气/飞沫传播——吸入感染者咳嗽或打喷嚏时排出的飞沫。例如结核病和流感。
Waterborne/foodborne transmission — ingestion of contaminated water or food via the faecal-oral route. Examples include cholera and typhoid.

水/食物传播——通过粪-口途径摄入受污染的水或食物。例如霍乱和伤寒。
Vector-borne transmission — transmission via intermediate organisms, usually insects, that carry the pathogen from one host to another. Examples include malaria (mosquito) and dengue fever.

媒介传播——通过中间生物（通常是昆虫）将病原体从一个宿主传播至另一个宿主。例如疟疾（蚊）和登革热。

The transmission route of a disease is closely linked to its prevention. For example, ensuring safe drinking water prevents cholera, while controlling mosquito populations prevents malaria.

疾病的传播途径与其预防措施密切相关。例如，确保安全饮用水可预防霍乱，而控制蚊虫种群则可预防疟疾。

8. Principles of Disease Prevention | 疾病预防的基本原则

Disease prevention aims to break the chain of infection at one or more points: eliminating the pathogen, blocking the transmission route, or protecting the susceptible host. This approach is summarised by the epidemiological triad: pathogen, host, and environment.

疾病预防的目的是在感染链的一个或多个环节将其阻断：消除病原体、阻断传播途径或保护易感宿主。这一方法可用流行病学三角来概括：病原体、宿主和环境。

There are several categories of prevention:

预防措施可分为若干类别：

Primary prevention — prevents the disease from occurring in the first place. Examples include vaccination, health education, and sanitation improvements.

一级预防——从源头上防止疾病发生。例如疫苗接种、健康教育和卫生设施改善。
Secondary prevention — aims to detect and treat the disease early to halt its progression and reduce transmission. Examples include screening programmes and early diagnosis.

二级预防——旨在早期发现和治疗疾病，以阻止其进展并减少传播。例如筛查项目和早期诊断。
Tertiary prevention — reduces complications and disability after the disease has developed. Examples include rehabilitation and supportive care.

三级预防——在疾病发生之后减少并发症和残疾。例如康复治疗和支持性护理。

At the individual level, prevention involves practising good hygiene, maintaining a healthy lifestyle to support the immune system, and completing vaccination schedules. At the population level, governments implement public health measures such as sanitation infrastructure, disease surveillance, and health education campaigns.

在个人层面，预防包括保持良好卫生习惯、维持健康生活方式以支持免疫系统，以及按时完成疫苗接种计划。在人群层面，政府实施公共卫生措施，如卫生基础设施、疾病监测和健康教育运动。

9. Vaccination & Immunisation | 疫苗接种与免疫

Vaccination is one of the most effective public health interventions for preventing communicable diseases. A vaccine contains a harmless form of the pathogen—either weakened (attenuated), killed (inactivated), or a component of it (such as a protein antigen or toxoid)—that stimulates an immune response without causing the full disease.

疫苗接种是预防传染病最有效的公共卫生干预措施之一。疫苗含有一种无害形式的病原体——减毒活疫苗、灭活疫苗，或病原体的一部分（如蛋白质抗原或类毒素）——可刺激免疫应答而不导致完整疾病。

When a person is vaccinated, their immune system produces memory cells (both B memory cells and T memory cells). If the same pathogen is encountered later, the immune system mounts a rapid, secondary immune response, producing large numbers of antibodies quickly and preventing illness.

当人接种疫苗后，免疫系统会产生记忆细胞（包括B记忆细胞和T记忆细胞）。如果之后遇到同种病原体，免疫系统会迅速发起二次免疫应答，快速产生大量抗体，从而防止发病。

Herd immunity occurs when a sufficiently high proportion of the population is vaccinated, such that the pathogen cannot spread easily because there are too few susceptible individuals. This provides indirect protection to those who cannot be vaccinated, such as infants or immunocompromised individuals.

群体免疫是指当人群中疫苗接种比例足够高时，易感个体太少导致病原体难以传播，从而为无法接种疫苗的人群（如婴儿或免疫功能低下者）提供间接保护。

The effectiveness of vaccination programmes depends on several factors, including the immunogenicity of the vaccine, vaccine coverage rates, and the presence of booster doses to maintain long-term immunity. Disruptions to vaccination programmes—as seen during the COVID-19 pandemic—can lead to resurgences of diseases like measles and polio.

疫苗接种计划的效果取决于多种因素，包括疫苗的免疫原性、疫苗覆盖率以及维持长期免疫力所需的加强针。疫苗接种计划的中断——如COVID-19疫情期间出现的状况——可能导致麻疹、脊髓灰质炎等疾病卷土重来。

10. Hygiene, Sanitation & Vector Control | 卫生、环境卫生与媒介控制

Improving hygiene and sanitation breaks the faecal-oral transmission route for diseases such as cholera. Key measures include providing clean, treated drinking water; proper sewage disposal; hand-washing with soap; and safe food handling and storage.

改善个人卫生和环境卫生可切断霍乱等疾病的粪-口传播途径。关键措施包括：提供经处理的清洁饮用水；妥善处理污水；用肥皂洗手；以及安全地处理和储存食物。

For vector-borne diseases, vector control is the primary prevention strategy. The table below summarises the main vector-control approaches.

对于媒介传播疾病，控制媒介是首要预防策略。下表总结了主要的媒介控制方法。

Method	Description	Example
Physical control	Removing or modifying breeding sites	Draining stagnant water, clearing vegetation
Chemical control	Using insecticides to kill vectors	Indoor residual spraying, larvicides
Biological control	Introducing natural predators or pathogens of the vector	Introducing fish that eat mosquito larvae
Personal protection	Barrier methods that prevent vector bites	Insecticide-treated bed nets, repellents

Integrated vector management (IVM) combines multiple approaches to reduce vector populations sustainably, slowing the development of insecticide resistance and minimising environmental impact.

综合媒介管理（IVM）结合多种方法可持续地降低媒介种群数量，减缓杀虫剂抗性的发展，并最大限度地减少对环境的影响。

11. Global Health Strategies | 全球健康策略

Many communicable diseases do not respect national borders, and global cooperation is essential for effective control. International organisations such as the World Health Organization (WHO) coordinate surveillance, provide guidance, and mobilise resources for disease prevention and control.

许多传染病不受国界限制，因此全球合作对于有效防控至关重要。世界卫生组织（WHO）等国际组织负责协调监测、提供指导，并调动资源用于疾病防控。

Key global strategies include the Expanded Programme on Immunisation (EPI), which aims to ensure universal access to essential vaccines; disease eradication programmes, such as the successful smallpox eradication campaign and the ongoing polio eradication initiative; and the Global Fund to Fight AIDS, Tuberculosis and Malaria, which finances prevention and treatment programmes in low-income countries.

关键全球策略包括：扩大免疫规划（EPI），旨在确保普及基础疫苗；疾病根除计划，如成功的天花根除运动和正在进行的脊髓灰质炎根除倡议；以及全球抗击艾滋病、结核病和疟疾基金，为低收入国家的预防和治疗项目提供资金。

Surveillance systems play a critical role by tracking disease incidence and outbreaks, enabling rapid responses to potential epidemics. Public health education empowers communities to adopt preventive behaviours, such as vaccination, safe sexual practices, and proper food hygiene.

监测系统通过追踪疾病发病率和疫情暴发发挥着关键作用，能够对潜在流行病做出快速响应。公共卫生教育赋权社区采纳预防行为，如疫苗接种、安全性行为和良好的食品卫生习惯。

Antimicrobial resistance (AMR) poses a growing threat to the control of communicable diseases. Misuse and overuse of antibiotics have led to the emergence of resistant strains—such as MDR-TB and MRSA—making infections harder to treat and increasing mortality rates. Responsible prescribing and adherence to treatment regimens are essential to preserve the effectiveness of existing drugs.

抗菌素耐药性（AMR）对传染病防控构成日益严重的威胁。抗生素的误用和过度使用催生了耐药菌株，如耐多药结核病和MRSA，使感染更难治疗，死亡率上升。负责任地开药和遵医嘱完成疗程对于保持现有药物的有效性至关重要。

12. Exam Summary | 考点总结

At AS level, students should be able to describe the characteristics of the four main pathogen types and the diseases they cause; explain transmission routes with reference to specific diseases; compare the structures of bacteria and viruses; and evaluate the social and biological factors that influence disease prevention.

在AS阶段，学生应能够描述四类主要病原体的特征及其引起的疾病；结合具体疾病解释传播途径；比较细菌和病毒的结构；并评估影响疾病预防的社会和生物因素。

Key revision points for this topic:

本主题的关键复习要点：

Cholera → bacterium, faecal-oral route, treated with ORT and antibiotics; prevention = clean water and sanitation.

霍乱 → 细菌，粪-口途径传播，用口服补液和抗生素治疗；预防=清洁水和卫生设施。
TB → bacterium, airborne route, affects lungs, long antibiotic course; prevention = vaccination (BCG) and early detection.

结核病 → 细菌，空气飞沫传播，侵犯肺部，长程抗生素治疗；预防=疫苗接种（卡介苗）和早期发现。
HIV/AIDS → virus, body-fluid transmission, destroys helper T cells; prevention = safe sex, needle exchange, ART.

艾滋病 → 病毒，体液传播，破坏辅助性T细胞；预防=安全性行为、针具交换、抗逆转录病毒治疗。
Malaria → protoctist, mosquito vector (Anopheles), cyclical fever from RBC rupture; prevention = vector control and bed nets.

疟疾 → 原生生物，按蚊传播，红细胞破裂导致周期性发热；预防=媒介控制和蚊帐。
Vaccination stimulates memory cells → secondary immune response → long-term immunity; herd immunity protects the unvaccinated.

疫苗接种刺激记忆细胞 → 二次免疫应答 → 长期免疫力；群体免疫保护未接种人群。

When answering exam questions on this topic, always state the pathogen type, the transmission route, the organ(s) affected, and the specific prevention or treatment measure. Use examples to illustrate your points, and be precise with terminology—for instance, distinguishing between ‘vector’ (an organism that transmits a disease) and ‘vehicle’ (an inanimate object that carries a pathogen).

在回答本主题的考试题目时，务必说明病原体类型、传播途径、受累器官以及具体的预防或治疗措施。用实例说明观点，并注意术语的精确使用——例如区分‘媒介’（传播疾病的生物）和‘载体’（携带病原体的无生命物体）。

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Communicable Diseases & Disease Prevention | 传染病与疾病预防