📚 Infectious Diseases | 传染病
Infectious diseases are disorders caused by pathogenic microorganisms, such as bacteria, viruses, fungi and protoctists. These diseases can be transmitted directly or indirectly from one person to another, or from an animal vector to a human. Understanding the biology of these pathogens, their modes of transmission and the body's defence mechanisms is essential for developing effective strategies to prevent and treat infections.
传染病是由细菌、病毒、真菌和原生生物等病原微生物引起的疾病。这些疾病可以直接或间接地从一个人传播给另一个人，或通过动物媒介传播给人类。理解这些病原体的生物学特性、传播方式以及机体的防御机制，对于制定有效的预防和治疗策略至关重要。
1. Introduction to Infectious Diseases | 传染病概述
An infectious disease occurs when a pathogen invades a host organism, multiplies and causes damage to tissues. The pathogen may release toxins, directly destroy host cells or trigger an excessive immune response. Not all microorganisms are pathogenic; many are harmless commensals. Disease symptoms often arise from the interaction between the pathogen's virulence factors and the host's immune responses.
当病原体侵入宿主、繁殖并损伤组织时，就会发生传染病。病原体可能释放毒素、直接破坏宿主细胞或引发过度的免疫应答。并非所有微生物都是致病的；许多是无害的共生菌。疾病的症状通常源于病原体毒力因子与宿主免疫应答之间的相互作用。
A key concept in A-Level Biology is the distinction between infectious and non-infectious diseases. Infectious diseases are caused by pathogens and can be transmitted, whereas non-infectious diseases, such as cancer or genetic disorders, are not caused by transmissible agents.
A-Level 生物学中的一个关键概念是区分传染性疾病与非传染性疾病。传染病由病原体引起并可以传播，而非传染性疾病（如癌症或遗传病）并非由可传播的因子引起。
2. Types of Pathogens | 病原体类型
Pathogens can be classified into four main groups: bacteria, viruses, fungi and protoctista (protozoa). Each group has distinct structural and reproductive characteristics that influence how they cause disease and how they can be controlled.
病原体可分为四大类：细菌、病毒、真菌和原生生物。每类具有不同的结构和繁殖特征，这决定了它们如何引起疾病以及如何被控制。
| Pathogen Type | Key Features | Example Disease |
|---|---|---|
| Bacteria | Prokaryotic, unicellular, peptidoglycan cell wall, reproduce by binary fission | Cholera, Tuberculosis |
| Viruses | Acellular, genetic material (DNA or RNA) inside a protein capsid, require host cells to replicate | HIV/AIDS, Influenza |
| Fungi | Eukaryotic, uni- or multicellular, cell walls made of chitin | Athlete's foot, Ringworm |
| Protoctista | Eukaryotic, unicellular, some are parasitic | Malaria (Plasmodium) |
Bacteria are typically treated with antibiotics that target specific prokaryotic structures, such as the cell wall, without harming human cells. Viruses, being non-living outside host cells, are not affected by antibiotics and require antiviral drugs that inhibit viral replication. Fungal infections are treated with antifungals, often targeting ergosterol in fungal membranes. Protoctist parasites like Plasmodium have complex life cycles requiring drugs that target different stages.
细菌通常用抗生素治疗，这些药物靶向特定的原核结构，如细胞壁，而不伤害人类细胞。病毒在宿主细胞外没有生命活性，不受抗生素影响，需要使用抑制病毒复制的抗病毒药物。真菌感染用抗真菌药治疗，常靶向真菌细胞膜中的麦角固醇。像疟原虫这样的原生生物寄生虫具有复杂的生命周期，需要用针对不同阶段的药物。
3. Transmission of Infectious Diseases | 传染病的传播途径
Understanding how pathogens spread is critical for disease control. Transmission can be direct or indirect. Direct transmission includes physical contact, droplet infection (coughing/sneezing) and vertical transmission from mother to child. Indirect transmission involves contaminated food or water (faecal-oral route), fomites (contaminated objects), airborne particles and vectors such as mosquitoes.
理解病原体如何传播对于疾病控制至关重要。传播可以是直接的或间接的。直接传播包括身体接触、飞沫传染（咳嗽/打喷嚏）以及母婴垂直传播。间接传播涉及受污染的食物或水（粪-口途径）、污染物（被污染的物体）、空气悬浮颗粒和媒介生物（如蚊子）。
Several classic diseases illustrate these routes: cholera spreads via contaminated water (faecal-oral), tuberculosis through airborne droplets, malaria through an insect vector (female Anopheles mosquito) and HIV through exchange of body fluids.
几种经典疾病说明了这些途径：霍乱通过受污染的水传播（粪-口途径），结核病通过空气飞沫传播，疟疾通过昆虫媒介（雌性按蚊）传播，艾滋病则通过体液交换传播。
4. Cholera: a Bacterial Disease | 霍乱：一种细菌性疾病
Cholera is caused by the bacterium Vibrio cholerae, a curved, Gram-negative rod. Transmission occurs when a person ingests food or water contaminated with faeces from an infected individual. The bacteria colonise the small intestine and secrete cholera toxin.
霍乱由细菌霍乱弧菌引起，它是一种弯曲的革兰氏阴性杆菌。当人摄入被感染者粪便污染的食物或水时，就会发生传播。细菌在小肠定殖并分泌霍乱毒素。
The toxin enters intestinal epithelial cells and activates the enzyme adenylate cyclase, causing a rise in cyclic AMP. This leads to the opening of chloride ion channels, and Cl⁻ ions flood into the lumen. Sodium ions follow, lowering the water potential of the lumen, so water moves out of the cells by osmosis. The result is profuse, watery diarrhoea, leading to severe dehydration and electrolyte loss.
毒素进入肠上皮细胞并激活腺苷酸环化酶，导致环AMP浓度升高。这促使氯离子通道开放，Cl⁻ 离子大量涌入肠腔。钠离子随之流出，降低了肠腔水势，因此水通过渗透作用从细胞中流出。结果出现大量水样腹泻，导致严重脱水和电解质流失。
Treatment involves oral rehydration therapy (ORT) using a solution of glucose and salts. Glucose stimulates the uptake of sodium ions via co-transport, which improves water absorption. Prevention relies on improved sanitation, clean water supplies and proper sewage treatment.
治疗包括使用含葡萄糖和盐的溶液进行口服补液疗法。葡萄糖通过协同转运促进钠离子吸收，从而改善水分吸收。预防依靠改善卫生设施、清洁供水及妥善处理污水。
5. Malaria: a Protoctist Disease | 疟疾：一种原生生物疾病
Malaria is caused by protoctists of the genus Plasmodium, with P. falciparum being the most lethal. The parasite has a complex life cycle involving a human host and a female Anopheles mosquito vector. When an infected mosquito bites, sporozoites enter the bloodstream and travel to the liver, where they multiply inside hepatocytes.
疟疾由疟原虫属的原生生物引起，其中恶性疟原虫致死率最高。该寄生虫具有复杂的生命周期，涉及人体宿主和雌性按蚊媒介。当受感染的蚊子叮咬时，子孢子进入血液并前往肝脏，在肝细胞内繁殖。
After a period, merozoites are released into the blood and invade red blood cells. Inside erythrocytes, they feed on haemoglobin, multiply and burst the cells synchronously, releasing more merozoites. This synchronised bursting causes the characteristic fevers and chills. Some parasites develop into gametocytes, which are taken up by another mosquito, completing the cycle.
一段时间后，裂殖子释放入血并侵入红细胞。在红细胞内，它们以血红蛋白为食，增殖并同步破裂细胞，释放更多裂殖子。这种同步破裂引发典型的发热和寒战。部分寄生虫发育为配子体，被另一只蚊子摄取，完成循环。
Control strategies include insecticide-treated bed nets, indoor residual spraying, prophylactic antimalarial drugs and, more recently, the RTS,S vaccine. Eradicating breeding sites of mosquitoes also reduces transmission. Drug resistance in Plasmodium, particularly to chloroquine, complicates treatment.
控制策略包括经杀虫剂处理的蚊帐、室内滞留喷洒、预防性抗疟药物以及最近问世的 RTS,S 疫苗。清除蚊子孳生地也能减少传播。疟原虫对氯喹等药物的耐药性使治疗复杂化。
6. Tuberculosis: a Bacterial Disease | 结核病：一种细菌性疾病
Tuberculosis (TB) is caused by Mycobacterium tuberculosis, an acid-fast bacillus. It spreads via airborne droplets when an infected person coughs or sneezes. The bacteria primarily infect the lungs, where they are engulfed by alveolar macrophages but can survive and multiply inside them, forming tubercles.
结核病由结核分枝杆菌（一种抗酸杆菌）引起。当感染者咳嗽或打喷嚏时，通过空气飞沫传播。细菌主要感染肺部，被肺泡巨噬细胞吞噬后却能存活并繁殖，形成结核结节。
Symptoms include a persistent cough (often with blood-tinged sputum), chest pain, fever, night sweats and weight loss. Infection may remain latent, but in immunocompromised individuals, it can become active. Diagnosis involves chest X-rays, sputum microscopy and tuberculin skin tests.
症状包括持续性咳嗽（常带血痰）、胸痛、发热、盗汗和体重减轻。感染可保持潜伏，但在免疫功能低下者体内可变为活动性。诊断方法包括胸部X光、痰液显微镜检查和结核菌素皮试。
TB treatment requires a prolonged course of multiple antibiotics (e.g., isoniazid, rifampicin, ethambutol) to prevent resistance. The BCG vaccine provides some protection, especially against severe childhood TB. Drug-resistant strains, such as MDR-TB and XDR-TB, pose a serious global health threat.
结核病治疗需要长期使用多种抗生素（如异烟肼、利福平、乙胺丁醇）以防耐药。卡介苗提供一定的保护，尤其预防儿童重症结核病。耐多药结核和广泛耐药结核等耐药菌株构成严重的全球健康威胁。
7. HIV/AIDS: a Viral Disease | 艾滋病：一种病毒性疾病
Human immunodeficiency virus (HIV) is a retrovirus that targets helper T cells (CD4⁺ lymphocytes), cells that are central to the adaptive immune response. The virus attaches to CD4 receptors and co-receptors on the T cell surface and fuses with the cell membrane, releasing viral RNA into the cytoplasm.
人类免疫缺陷病毒（HIV）是一种逆转录病毒，攻击辅助T细胞（CD4⁺淋巴细胞），这些细胞是适应性免疫应答的核心。病毒附着于T细胞表面的CD4受体和共受体，与细胞膜融合后将病毒RNA释放到细胞质中。
The enzyme reverse transcriptase transcribes RNA into DNA, which is then integrated into the host genome by integrase. The infected cell can lie dormant or actively produce new viruses. Over time, the destruction of helper T cells leads to severe immunodeficiency, known as acquired immunodeficiency syndrome (AIDS). Patients become vulnerable to opportunistic infections and certain cancers.
逆转录酶将RNA转录为DNA，然后由整合酶将其整合到宿主基因组。受感染的细胞可潜伏休眠或活跃产生新病毒。随着时间的推移，辅助T细胞的破坏导致严重免疫缺陷，即获得性免疫缺陷综合征（艾滋病）。患者极易受到机会性感染和某些癌症的侵袭。
HIV is transmitted through blood, semen, vaginal fluids and breast milk. There is no cure, but antiretroviral therapy (ART) can control viral replication, maintain immune function and reduce transmission. Prevention strategies include safe sex practices, needle exchange programs and prophylactic drugs for high-risk individuals.
HIV通过血液、精液、阴道分泌物和母乳传播。虽然无法治愈，但抗逆转录病毒疗法可控制病毒复制、维持免疫功能并减少传播。预防策略包括安全性行为、针具交换计划及高危人群的预防性用药。
8. Non-specific Defence Mechanisms | 非特异性防御机制
The body employs several non-specific, innate defences that act rapidly against any pathogen. The first line of defence includes physical and chemical barriers. Intact skin forms an impermeable barrier, and sebum contains antimicrobial substances. Mucous membranes trap pathogens, and cilia in the respiratory tract sweep them away. Stomach acid (HCl) destroys many ingested microbes, and lysozyme in tears and saliva breaks down bacterial cell walls.
机体拥有多种快速起效的非特异性天然防御机制。第一道防线包括物理和化学屏障。完整的皮肤形成不可渗透的屏障，皮脂含有抗微生物物质。粘膜截留病原体，呼吸道的纤毛将其清除。胃酸（HCl）破坏许多吞入的微生物，泪液和唾液中的溶菌酶分解细菌细胞壁。
The second line of defence involves cellular and chemical responses. Phagocytes (neutrophils and macrophages) recognise, engulf and digest pathogens by phagocytosis. The inflammatory response increases blood flow and recruits immune cells to the site of infection. Interferons are proteins released by virus-infected cells that inhibit viral replication in neighbouring cells. Fever raises body temperature, which may slow pathogen growth and enhance immune activity.
第二道防线涉及细胞和化学应答。吞噬细胞（中性粒细胞和巨噬细胞）通过吞噬作用识别、摄取和消化病原体。炎症反应增加血流量并募集免疫细胞到感染部位。干扰素是病毒感染的细胞释放的蛋白质，能抑制邻近细胞的病毒复制。发热升高体温，可减缓病原体生长并增强免疫活性。
9. Specific Immune Response | 特异性免疫应答
The specific (adaptive) immune response targets particular pathogens with high specificity and generates immunological memory. It involves two main branches: humoral immunity mediated by B lymphocytes and antibodies, and cell-mediated immunity driven by T lymphocytes.
特异性（适应性）免疫应答以高度特异性靶向特定病原体，并产生免疫记忆。它包括两个主要分支：由B淋巴细胞和抗体介导的体液免疫，以及由T淋巴细胞驱动的细胞介导免疫。
When a pathogen's antigens are presented on the surface of antigen-presenting cells (dendritic cells, macrophages), helper T cells with matching receptors become activated. Activated helper T cells stimulate B cells specific to the same antigen. B cells undergo clonal expansion and differentiate into plasma cells, which secrete large amounts of antibodies. Antibodies bind to antigens, neutralising toxins, agglutinating pathogens and marking them for phagocytosis (opsonisation).
当病原体的抗原呈现在抗原呈递细胞（树突状细胞、巨噬细胞）表面时，具有匹配受体的辅助T细胞被激活。活化的辅助T细胞刺激识别相同抗原的B细胞。B细胞进行克隆扩增并分化为浆细胞，分泌大量抗体。抗体与抗原结合，中和毒素、凝集病原体并标记它们以供吞噬（调理作用）。
Cell-mediated immunity involves cytotoxic T cells that kill infected body cells. They recognise viral or tumour antigens displayed on MHC class I molecules and release perforin and granzymes to induce apoptosis. Both B and T cells produce long-lived memory cells, enabling a faster, stronger response upon re-exposure.
细胞介导免疫涉及杀伤受感染细胞的细胞毒性T细胞。它们识别MHC I类分子上展示的病毒或肿瘤抗原，并释放穿孔素和颗粒酶诱导细胞凋亡。B细胞和T细胞都产生长寿的记忆细胞，使再次暴露时能够产生更快、更强的应答。
10. Vaccination and Herd Immunity | 疫苗接种与群体免疫
Vaccination artificially induces active immunity by introducing a harmless form of a pathogen or its antigens into the body. This triggers a primary immune response, including the production of memory cells. Upon later exposure to the actual pathogen, the secondary response is rapid and potent, preventing disease.
疫苗接种通过将无害形式的病原体或其抗原引入体内，人工诱导主动免疫。这触发初次免疫应答，包括记忆细胞的产生。再次接触真实病原体时，二次应答迅速而强烈，从而预防疾病。
Vaccines may be live attenuated, inactivated, subunit or toxoid, depending on the disease. For example, the MMR vaccine contains live attenuated measles, mumps and rubella viruses. Herd immunity occurs when a sufficiently high proportion of a population is immune, making person-to-person spread unlikely and protecting vulnerable individuals who cannot be vaccinated.
根据疾病不同，疫苗可以是减毒活疫苗、灭活疫苗、亚单位疫苗或类毒素疫苗。例如，MMR疫苗含有减毒的麻疹、腮腺炎和风疹活病毒。群体免疫是指当人群中足够高的比例获得免疫时，人际传播难以发生，从而保护无法接种疫苗的易感个体。
Global vaccination programs have eradicated smallpox and dramatically reduced the incidence of polio, measles and tetanus. The estimated herd immunity threshold depends on the basic reproduction number (R₀); for measles, about 95% coverage is required.
全球疫苗接种计划已消灭天花，并大幅降低了脊髓灰质炎、麻疹和破伤风的发病率。群体免疫阈值取决于基本再生数（R₀）；例如麻疹需要大约95%的覆盖率。
11. Antibiotics and Antibiotic Resistance | 抗生素与抗生素耐药性
Antibiotics are chemical substances that kill or inhibit the growth of bacteria. They work by targeting structures and processes unique to prokaryotes, such as cell wall synthesis (penicillin), protein synthesis at 70S ribosomes (tetracyclines) and nucleic acid replication. Because viruses lack these targets, antibiotics are ineffective against viral infections.
抗生素是杀死或抑制细菌生长的化学物质。它们通过靶向原核生物特有的结构和过程起作用，如细胞壁合成（青霉素）、70S核糖体蛋白质合成（四环素类）和核酸复制。由于病毒缺乏这些靶标，抗生素对病毒感染无效。
Antibiotic resistance arises through genetic mutations or horizontal gene transfer (via plasmids) that confer resistance. When antibiotics are used, susceptible bacteria die, but resistant ones survive and proliferate by natural selection. These resistant strains, such as methicillin-resistant Staphylococcus aureus (MRSA), are difficult to treat and can spread in hospitals.
抗生素耐药性通过基因突变或水平基因转移（通过质粒）产生。使用抗生素时，敏感菌死亡，但耐药菌存活并通过自然选择增殖。这些耐药菌株，如耐甲氧西林金黄色葡萄球菌（MRSA），难以治疗并在医院内传播。
To minimise resistance, it is essential to use antibiotics only when necessary, complete the prescribed course to prevent survival of partially resistant bacteria, and avoid use in animal feed. Developing new antibiotics and alternative therapies is an ongoing challenge.
为了减少耐药性，必须只在必要时使用抗生素，完成整个疗程以防止部分耐药菌存活，并避免在动物饲料中使用抗生素。研发新抗生素和替代疗法是一项持续的挑战。
12. Controlling the Spread of Infectious Diseases | 控制传染病的传播
Effective disease control integrates multiple approaches. At the public health level, provision of clean water and sanitation systems interrupts faecal-oral transmission. Proper sewage treatment prevents cholera and other diarrhoeal diseases. Personal hygiene, including handwashing, reduces contact spread. Airborne infections like TB are controlled by ventilation and isolation of cases.
有效的疾病控制整合了多种方法。在公共卫生层面，提供清洁水和卫生系统可阻断粪-口传播。妥善的污水处理能预防霍乱和其他腹泻病。个人卫生（包括洗手）减少接触传播。像结核病这样的空气传播感染通过通风和隔离病例来控制。
Vector control is crucial for malaria and other vector-borne diseases; draining stagnant water, using insecticides and bed nets all reduce mosquito populations. Vaccination programs build herd immunity and can eliminate diseases. Surveillance and rapid response to outbreaks prevent epidemics from spreading. Education campaigns inform the public about transmission and protection.
媒介控制对于疟疾等虫媒疾病至关重要；排干积水、使用杀虫剂和蚊帐均可减少蚊子数量。疫苗接种计划建立群体免疫并能消灭疾病。疫情监测和快速反应可防止流行病扩散。宣传教育向公众传播相关知识和防护方法。
Globally, organisations such as the WHO coordinate efforts against infectious diseases. Antimicrobial stewardship programs aim to preserve the effectiveness of existing drugs. Emerging threats like pandemic influenza or novel coronaviruses require international collaboration, data sharing and the rapid development of diagnostics and vaccines.
在全球范围内，世卫组织等机构协调应对传染病的工作。抗微生物药物管理项目旨在保持现有药物的有效性。大流行性流感或新型冠状病毒等新威胁需要国际合作、数据共享以及快速开发诊断方法和疫苗。
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