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The epidemiology of infectious diseases is concerned with the study of the occurrence，development，and distribution of infectious diseases within a population with the ultimate goal of preventing and controlling their spread.The epidemiological process involves three essential components：the source of infection，the mode of transmission，and the susceptible population. Pathogens such as bacteria，viruses， mycoplasmas ，and parasites are the causative agents of infectious diseases and can be transmitted via respiratory transmission， gastrointestinal transmission，insect-borne transmission，and body fluid transmission.
Epidemiology of infectious diseases is a practical science aimed at reducing the impact of pathogens on public health and well-being.Traditional epidemiology of infectious diseases has focused primarily on the measurement of biological，environmental，and behavioral risk factors at the individual level.However，recent research has shown that infectious diseases are not only public health problems but also social problems，such as the case of acquired immunodeficiency syndrome （AIDS）.Researchers in the field are increasingly interested in elucidating the social context of infectious disease transmission，risk factors，and health inequities .
The term "infectious disease" refers to a range of illnesses caused by pathogens that can be transmitted between humans，between animals，or from animals to humans.Despite the significant progress made in the prevention and control of infectious diseases globally，such as the eradication of smallpox，and the effective control of diphtheria，pertussis，tetanus，polio，and plague，the current threat of infectious diseases remains pressing.In recent years，there has been a resurgence of syphilis，gonorrhea，and schistosomiasis，as well as ongoing high rates of tuberculosis and viral hepatitis.Additionally，the emergence of new infectious diseases such as Ebola hemorrhagic fever，legionnaires disease，AIDS，Lyme disease，highly pathogenic avian influenza，severe acute respiratory syndrome，and COVID-19 has added to the dual threat of both old and new infectious diseases.
According to the intensity and breadth of the epidemic process，the epidemiological characteristics of infectious diseases can be classified as sporadic，epidemic，pandemic and outbreak.A sporadic refers to an incidence level of an infectious disease that is consistent with the general level of the calendar year，without any obvious correlations between cases in terms of time and place of onset.An epidemic is an incidence of an infectious disease in a specific area that exceeds the incidence level of the same period in the calendar year，with clear temporal and spatial links between cases.A pandemic is characterized by a rapid spread of an infectious disease across wide geographical areas，including across provincial，national，or even continental boundaries，and with a greater intensity than an epidemic.An outbreak refers to the sudden appearance of a large number of patients with similar symptoms in a short period of time in a localized area or unit，with a common source of infection or transmission route.
The occurrence of infectious diseases in a population is dependent on the convergence of three fundamental conditions，referred to as the three links of the epidemic process.These three links in disease occurrence include the source of infection，the method of transmission，and the presence of a susceptible population.When these three components coexist and interact，they can result in the outbreak of infectious diseases.
The source of infection is the body of the pathogen growth，reproduction and discharge of pathogens of people or animals，including patients with infectious diseases，carriers of pathogens and infected animals.Patients with infectious diseases are important sources of infection，such as diphtheria，tuberculosis and other respiratory infectious diseases when patients cough can discharge a large number of pathogens，increasing the chances of susceptible people being infected.The significance of a patient as a source of infection can be influenced by several factors，including the number of pathogens excreted，the virulence of the pathogens，and the range of activity of the patient.In the progression of an infectious disease，the patient's condition can typically be divided into three phases：the incubation period，the clinical symptoms period，and the recovery period，each with its own varying levels of pathogen excretion .In addition to infected individuals，carriers of pathogens such as bacteria，viruses，mycoplasma and spirochetes also play a role in the transmission of infectious diseases.Carriers of pathogens can be divided into three categories：latent carriers of infectious diseases，recovering carriers，and healthy carriers.Infected animals can also serve as a source of infection for certain human infectious diseases.The pathogens of some diseases are mainly transmitted among animals in nature and can be transmitted to humans under certain conditions，resulting in diseases called natural epidemic diseases or zoonotic diseases，such as plague，brucellosis，forest encephalitis，leptospirosis，schistosomiasis，rabies，etc.
The transmission route refers to the process by which pathogens are transmitted from an infected source to a susceptible host.It encompass es the entire journey that the pathogen undergoes in the external environment after being discharged from the infectious source and before invading a new host.The transmission route of an infectious disease can be complex and can vary based on multiple factors.Each infectious disease can be transmitted by one or more routes of transmission.
In general，transmission routes are classified into two categories： vertical transmission and horizontal transmission.Vertical transmission，also known as mother-to-child transmission or perinatal transmission，refers to the transfer of pathogens from a mother to her offspring during the perinatal period.This type of transmission can be further classified into three subcategories：transplacental transmission，upstream transmission，and transmission during childbirth.Horizontal transmission，on the other hand，refers to the transmission of pathogens from one person to another.This type of transmission can occur through various routes，including airborne transmission，contact transmission，waterborne transmission，foodborne transmission，and vector-borne arthropod transmission.
Airborne transmission is a crucial mode of transmission for respiratory infectious diseases（e.g.，measles，chickenpox，influenza，etc.）and includes droplets，droplet nuclei，and dust.The characteristics of airborne infectious diseases include extensive dissemination ，facile achievement of the transmission route，and a high rate of incidence.In addition，airborne infectious diseases are seasonally distinct，commonly occurring in winter and spring；have a cyclical increase in incidence in populations without immunization and prophylaxis ；and tend to occur in areas with crowded living and high population density.
Transmission by contact can be divided into direct contact transmission and indirect contact transmission according to whether the pathogen stays in the external environment before invading the susceptible host organism after leaving the infectious source.
Direct contact transmission occurs when the pathogen is transmitted directly from the source of infection to the susceptible host without the involvement of external factors，such as in the case of rabies or sexually transmitted diseases，etc.On the other hand，indirect contact transmission takes place as a result of contact with objects contaminated with pathogens，which is often the case in many intestinal，respiratory，and skin infectious diseases.This mode of transmission is usually caused by contact with everyday objects contaminated with pathogens.
Waterborne transmission is the main route of transmission for many intestinal infectious diseases （such as cholera，typhoid，dysentery，hepatitis A，etc.）and certain parasitic diseases （such as schistosomiasis，leptospirosis，etc.）.Waterborne infectious diseases have two transmission routes，namely drinking water contaminated with pathogens and contact with water contaminated with pathogens.Waterborne diseases are often epidemic outbreaks，and their epidemiological characteristics are：case distribution and water supply range consistent with a history of drinking the same water source or contact with infected water；except for nursing infants，the onset of disease without age and gender differences，more cases of drinking raw water；in the case of water sources are often contaminated，cases often continue to appear，strengthen the treatment of infected water and personal protection，can control the occurrence of cases；acute onset is often seasonal，regional and occupational，with patients mostly seen in water network areas，during the rainy season and after disasters，and the onset is mostly seen in occupational groups exposed to infected water.Transmission through contact with pathogen-infected water is usually due to the invasion of pathogens （e.g.，schistosomes and leptospira ）through the skin and mucous membranes when people come into contact with water containing specific pathogens.
Foodborne transmission is a key mode of transmission for a range of intestinal infectious diseases，including typhoid fever and bacterial dysentery，as well as certain parasitic diseases，such as pig tapeworm.The hallmark features of foodborne infectious diseases are：the presence of a history of consuming contaminated food，with only those who consume the contaminated food developing the disease；a short incubation period，which is often seen during the summer and fall；outbreaks that can occur among diners and can be effectively curtailed when the supply of contaminated food is discontinued；moreover，when the food is repeatedly contaminated，persistent outbreaks and epidemics can occur.
Vector-borne arthropod transmission，also known as insect-borne transmission，refers to infectious diseases that are transmitted through the mechanical carrying or biting of blood by vector arthropods，such as mosquitoes，flies，fleas，cicadas，mites，and cockroaches.This mode of transmission is a major route of transmission for a range of intestinal infectious diseases，zoonotic diseases，and parasitic diseases.Vector-borne transmission can be divided into two categories：biological and mechanical.
Biological transmission，also known as blood-sucking transmission，occurs when the pathogen enters the arthropod host，where it develops and reproduces，before being transmitted to susceptible individuals.Diseases transmitted through this mode include plague，epidemic B encephalitis，forest encephalitis，malaria，typhus，and filariasis.Mechanical transmission means that some arthropods （such as flies and cockroaches）can carry pathogens，but the pathogens cannot develop and reproduce in their bodies，and only contaminate food or eating utensils through contact，regurgitation or discharge of pathogens with feces when foraging，and people are infected by ingesting contaminated food or using contaminated eating utensils，such as typhoid fever and dysentery.
The term "susceptible individuals" refers to those individuals who lack immunity to a specific pathogen and are thus at risk of infection.For example，individuals who have not had measles or received measles vaccination are considered susceptible to the disease.The level of susceptibility in a population is referred to as "population susceptibility".It's determined by the proportion of the population that lacks immunity to the pathogen.A high level of herd immunity within a population reduces the level of population susceptibility，while a low level of herd immunity increases it.The susceptibility level of a population can be evaluated by analyzing past disease prevalence，vaccination status，and antibody level test results of the population for the specific disease in question.
pathogen 病原体
Pathogens are the source of many infectious diseases.
mycoplasma 支原体；霉形体
Mycoplasma pneumonia in children accounts for 10% to 40% of community-acquired pneumonia in children.
parasite 寄生生物；寄生虫；寄生植物；依赖他人过活者
Freshwater fish must be cooked to prevent parasites .
respiratory 呼吸的
Respiratory transmitted diseases are most likely to appear in winter and spring.
gastrointestinal 胃肠的
The most common gastrointestinal symptom of chronic gastritis is indigestion.
immunodeficiency 免疫缺陷
Opportunistic infections are often seen in patients with immunodeficiency .
elucidate 阐明；解释；说明
Please elucidate the reasons for your ridiculous decision.
context 上下文；语境；（事情发生的）背景，环境，来龙去脉
The cultural context of smoking control is a very important area of research.
inequity 不公正的事；不公正；不公平
Different regional levels of economic development bring health inequity.
significant 重要的，有重大意义的；显著的，值得注意的
Environmental pollution remains a significant global problem.
eradication 根除，消（扑）灭
The eradication of infectious diseases is not an easy task .
resurgence 复苏；复兴；重新抬头
The resurgence of sexually transmitted diseases poses a serious public health challenge.
consistent 一致的；始终如一的；连续的；持续的
A person's behavior is basically consistent with his principles.
temporal 时间的；世俗的；世间的；现世的
The epidemic of infectious disease should be analyzed from different temporal perspectives.
outbreak （暴力、疾病等坏事的）暴发，突然发生
Spoiled food led to this outbreak of food poisoning.
fundamental 十分重大的；根本的；基础的；基本的
The current fundamental route of HIV transmission is unprotected sexual contact.
components 组成部分；成分；部件
The precise relationship of cardiac components is a guarantee of cardiac function.
discharge 释放；排出；履行；流出
The sneeze of a person with a respiratory infection can discharge huge amounts of pathogens.
incubation 孵（卵）；孵化；（传染病的）潜伏期；（细菌等的）繁殖
The illness has an incubation period of up to 7 days.
excretion 排泄，排泄物；分泌，分泌物
Urine is an important excretion that monitors the health of the body.
zoonotic 动物传染病的；人畜共患病的
Rabies is a zoonotic disease that is transmitted between animals and humans.
susceptible 易受影响；敏感；过敏；好动感情的；善感的
Weak people are more susceptible to contract respiratory infections.
encompass 包含，包括，涉及（大量事物）；包围；围绕；围住
The transmissions of HIV encompass sexual，blood and mother-to-child transmission.
vertical 竖的；垂直的；直立的；纵向的
Mother-to-child transmission of syphilis is a vertical transmission.
horizontal 水平的；与地面平行的；横的
Please place the armrests in a horizontal position to ensure safety when landing the aircraft.
extensive 广阔的；广大的；大量的；广泛的；广博的
There are extensive galaxies and stars in the universe.
dissemination 传播；散播；
Most science materials written for public dissemination should be easy to understand.
prophylaxis （疾病）预防
Pre-exposure prophylaxis can reduce HIV transmission in high-risk populations.
contaminated 污染；弄脏；玷污，毒害，腐蚀
Drinking water near this gold mine has been contaminated with mercury.
intestinal 肠道的；肠的
Parasites in the intestinal tract can cause serious health problems.
leptospira 钩端螺旋体；细螺旋体
Leptospirosis is a zoonotic disease caused by pathogenic species of leptospira .
vaccination 接种疫苗；注射疫苗；种痘
Vaccination is very effective as a method of preventing smallpox.
acquired immunodeficiency syndrome 获得性免疫缺陷综合征
Acquired immunodeficiency syndrome is a serious infection disease in which the body's immune system is destroyed by human immunodeficiency virus，resulting in a combination of symptoms such as opportunistic infections and tumors that can eventually lead to death.
Acquired immunodeficiency syndrome was first seen in gay men in the United States，and the leading cause of death is Pneumocystis carinii pneumonia or Kaposi sarcoma.
incubation period 潜伏期
Incubation period is the period between infection and the appearance of symptoms of the disease.
There is usually an incubation period of 1-10 years after HIV infection，and infected people often have no obvious symptoms.
vertical transmission 垂直传播
Vertical transmission is a transmission route of an agent from an individual to its offspring，that is，from one generation to the next.
There are currently technologies that can be applied to stop the vertical transmission of HIV and prevent infection in infants.
intestinal infectious diseases 肠道传染病
Infectious diseases in which the pathogen invades the intestine through the mouth and can excrete pathogens from the feces，including cholera，bacterial dysentery，typhoid，paratyphoid，viral hepatitis，etc.
Intestinal infectious diseases are most prevalent in young children aged 1-4 years.
population susceptibility 人群易感性
Population susceptibility is the degree of susceptibility to an infectious disease in a population.A person who lacks specific immunity to an infectious disease and is susceptible to the disease is called a susceptible person to that infectious disease.
The high population susceptibility to a novel virus requires us to keep a close eye on its epidemiological situation.
Task 1：Vocabulary Application
Fill in the blanks with the words given below.Change the form where necessary.
Candidate words：
consistent ； inequity ； contaminated ； extensive ； parasites ； vaccination ； zoonotic ； susceptible ； vertical ； resurgence
1.Health ___________ among immigration is a common problem globally，especially in the distribution of health resources.
2.Each year，roughly one in six people in this poor area of the African country gets sick from eating ___________ food.
3.The ___________ axis of the graph generally has a clear scale，indicating the frequency.
4.The medical community needs to be alerted to the ___________ of diseases such as tuberculosis.
5.It is unreasonable that what you say is not ___________ with what you do.
6.He writes very good essays，mainly because he has an ___________ vocabulary.
7.There are ___________ in raw meat and fish as well as raw vegetables.
8.Young people are perhaps most ___________ by advertisements and buy useless items.
9.Control of ___________ diseases is one of the important tasks in the field of public health.
10.People's willingness of ___________ is often related to their level of health awareness and perceived threat of disease.
Task 2：Writing
For this part，you are allowed 30 minutes to write a composition on the topic：HIV/AIDS.You should write at least 120 words，and base your composition on the outline below：
1）What is HIV/AIDS？
2）How is HIV transmitted in the population？
3）What are the prevention and control strategies for HIV/AIDS？
Task 3：Oral Presentation
From your point of view，explain the necessity and urgency of HIV/AIDS prevention among the youth population.