【正文】 t infectivity period. Epidemiologic features Transmission of pathogenic organisms to other people, directly or indirectly, may lead to an epidemic finally. The epidemic of disease can happen through import (cholera) or origin from local area or home. General features: incidence of a disease in age, sex, season and geographic distribution. Incidence: cases in per 100,000 population per year. According to epidemic scale there are 4 type of epidemic: l Sporadic occurrence: the incidence of municable diseases is around the median incidence in past 5 years.l Epidemic: the incidence of municable diseases is significantly higher than the median of incidence of the disease in past 5 years.l Epidemic outbreak: A large number of cases occur within a short period by a same source of infection and a same route of transmission.l Pandemic: Epidemic of municable diseases spreads across the border of a country or a continent.Postinfection immunity Specific immunity acquire after covert infection or overt infection which can be detected by immunoassay. The immunity lasting vary in different disease, usually occurs after recovery of unicabte diseases.Clinical CharacteristicsDifferent stages of the clinical coursesl Incubation period: Incubation period is the period between the invasion of pathogens and the onset of disease. It is useful for diagnosis and deciding how long to isolate people who ever closely contact with patient.l Prodromal stage: The period is between the onset of disease and the period of apparent clinical manifestation. This stage may be absent in some diseases and nonspecific, Such as fever, headache, fatigue, anorexia, myalgia so on. It often lasts for 13 days.l Stage of apparent clinical manifestation., The period in which most of the typical symptoms and signs occur. Some infectious diseases are lack of this stage directly form prodromal stage into convalescent stage, so called “ abortive type”.l Convalescent stage: The period in which the pathological process nearly end and most of the clinical manifestations are subsiding. However there are pathologic changes ,abnormal biochemical test, municability.l Relapse: Initial symptoms and signs reoccur during the convalescent stage due to pathogen remained in tissue remultiplying, only in certain diseases (typhoid fever).l Recrudescence. Fever reappears when body temperature fall but is not normal. Major manifestationl FeverFever is often regarded as the cardinal feature of infection, but not all febrile illnesses are infections, and not all infectious diseases present with fever. The fever pattern may occasionally be helpful。 many infectious diseases have a characteristic fever.Clinical course of fever: A Effervescence: Climbing up of body temperature, may be gradual or sudden.B Fastigium (fullbrown stage): Body temperature maintains at high level for several days.C Defervescence: Body temperature e down suddenly (crisis) or gradually.Pattern of fever:A Sustained fever: Temperature 39oC and difference of temperature loC within 24 hours. (typhoid, typhus).B Remittent fever : The lowest temperature above normal and difference of temperature loC within 24 hours. (remission stage of typhoid).C Intermittent (septic) fever: Fluctuation of temperature between high fever and normal temperature (septicemia).D Relapsing fever: Fever for a few days alternating with normal temperature for a few days.E Undulant fever: fever fluctuates for weeks or months alternating with normal temperature.F Saddle type fever: There are interval of 1 day remission between two waves of fever for a few days (dengue fever).l Skin rash (eruption): Skin rashes are mon features of infectious diseases, and the ears, eyes, mouth and throat should also be inspected. The weight should be place on its pattern, distribution, time and order of appearance. For example, rash of chicken pox appears on fist day and rash of typhoid on sixth day。 maculopapular rash seen in measles, rubella。 petechia in epidemic hemorrhagic fever, vesiculopustula rash in herpes simplex, Zona, urticaria in hepatitis, eschar in scrub typhus.l Toxemic symptoms: When microorganism multiple in tissue, their productions and toxin are released into blood and cause a series of symptoms: general malaise, anorexia, muscle and joint pains, mental symptoms, even progress multiple organs failure.l Bacteremia: The presence of living organisms in the blood can occur in healthy people without causing symptoms.l Septicemia:Bacteria multiple in blood and produce toxin lead to a series of sympotoms above description. Septicaemia can be plicated by metastatic septic lesions in organs or tissues. Examples include pneumococcal pneumonia and meningococcal meningitis. Circulatory failure of the septic shock syndrome is the most dangerous plication of septicaemia and is caused by Grampositive, Gramnegative or fungal organisms. l Monocyticmacrophagic system reactio:Monocyticmacrophagic system proliferate, congest and lead to splenomegaly, hepatomegaly, lymphadenopathy.Clinical types: The clinical pattern can be classified into fulminant, acute, subacute, chronic。 mild, moderate, severe。 typical and atypical. DIAGNOSIS OF COMMUNICABLE DISEASE Early diagnosis is useful for isolating and treatment of the patient. Confirming diagnosis should be based on history, clinical manifestations, epidemical history, and investigations.Clinical manifestationsInfectious diseases can affect any organ or system, and can cause a wide variety of symptoms and signs. Historytaking and thorough examination covering all systems is required and should aim to identify the site(s) of infection, and also the likely causative organism(s).Epidemiological historyA detailed history is taken with specific questions about epidemiological risk factors for infection. These are based on the sources of infection and routes of transmission discussed above.l Trave